Hot Rod Forum banner
1 - 20 of 29 Posts

·
Registered
Joined
·
456 Posts
Discussion Starter · #1 ·
My non-assisted brake system uses two Tilton 74 series MCs and a balance bar, 7/8" for the front brakes and 3/4" for the rear. Trying to improve my stopping power, I started testing the front line pressure. The max I can get at a front caliper is a little over 500 psi and I should get 600 with a pedal ratio of 6:1. I adjusted the balance bar by a significant amount just to see what happens, basically 1/4" to favour the front brakes. The pressure did NOT go up, but stayed the same even though it ought to have gone up to 750 psi.

There are no external leaks and it doesn't seem like there are any internal leaks as the pedal does not continue to drop. There is no interference with pedal movement, i.e doen't hit the floor or anything.

The balance bar is not binding up and is perpindicular to the cylinders under full pedal pressure (front starts out with cylinder rod about 1/4" longer than the rear to allow more initial fluid transfer as the front calipers are larger than the rear ones).

Below is a picture of the set-up. I'm starting to suspect the front MC is internally leaking even though the pedal doesn't seem to be dropping. Any thoughts?

 

·
Race it, Don't rice it!
Joined
·
9,686 Posts
What calipers and what applications are we talking here?
Where di you get that 600/750 number?
What type of pads? Compound?
Weight of car?

Keep in mind the balance bar doesn't increase system pressure, only it's bias of total pressure front/back.
For whats it worth, I don't care the brand those bias bar always need work to be right.
 

·
Race it, Don't rice it!
Joined
·
9,686 Posts
Do you know the rear piston size? I'm trying to do the some math.
I'm not familiar with those and google isn't helping me much.
Also, When you said "more stopping power" do you mean that as too shorten stopping distance? or can you be more specific?
Whats the current front/rear bias and what bias are you looking for?
What pad compounds are you using now?
Are you racing or just for street use?
Whats the weight of the car and tire size?
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #5 ·
Jag rear piston dia. is 43 mm.

More stoppipng power means I can't lock up the tires, so it takes too long a distance to stop. Not that one wants to, but if they can be locked up it indicates the brake system is more thna adequate.

Front/rear bias takes quite a bit of calculation as not only are the front and rear MCs different diameters, but so are the caliper pistons and rotor diameters. At least my tires are the same diamter or there'd be abother factor to consider. A good point to aim for is about 70% front braking. I've done all the math aiming for 0.9 G's of braking. It wasn't that hard but took a lot of calculations. I'll print it out if I can ever find my notebook I wrote it all down in.

Pad compound? Not sure, but an average coefficient of friction is around 0.35.

Just street use.

Car weight is 2500 lbs and tire are 26" in diameter.
 

·
Registered
Joined
·
302 Posts
It is possible that you have air in the rear brake system.

In order for the balance bar to work one of the master cylinders has to be against a non-conpressable resistance. If the rear has air the force will simply be used to compress the gas. This would explain why your values didn't change.

Also, a little quick math says you are using around 120 lbs of force on the pedal to get 600 psi with a 50/50 split. A 60% bias would only increase your pressure to approximately 720 psi with a .875 master. You may not be able to get more without going to a smaller bore or putting out a lot more leg effort while staying within Tilton's limits for adjustment.

It is entirely possible that my math is wrong. I was working backwards from the numbers you gave.

A 13/16 bore would give approximately 700 psi at a 50/50 split and 830 with a 60% bias.
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #7 ·
It is possible that you have air in the rear brake system.

In order for the balance bar to work one of the master cylinders has to be against a non-conpressable resistance. If the rear has air the force will simply be used to compress the gas. This would explain why your values didn't change.
I've thought of that, however when the brakes are applied both cylinder tods stop a little ways in with lots of stroke left over. The balance bar perpindicular to the cylinders as they ought to be. If air was in the rear system, then I would expect the rear brake rod would keep on going.

Also, a little quick math says you are using around 120 lbs of force on the pedal to get 600 psi with a 50/50 split. A 60% bias would only increase your pressure to approximately 720 psi with a .875 master. You may not be able to get more without going to a smaller bore or putting out a lot more leg effort while staying within Tilton's limits for adjustment.
Yes, your math is the same as mine which is why getting only a little over 500 psi puzzles me.

The whole thing seems impossible, and when that happens it's usually because something was incorrectly assumed or was wrong in the presentation of the problem. Based on that I did find something wrong. If I hold the brakes down as hard as I can long enough, I can feel the pedal sink ever so slowly. So slowly I never noticed it during the tests or while driving. Now the pressure should be up to snuff when the brakes are first applied and then drop slowly, but that's not the case. It goes right to 500 psi or a little over and just stays there.

At any rate, I will be changing out both cylinders sometime soon. Maybe that will make a bigger difference than I expect.

ps: Oh, I did bleed the brakes again today. For a moment I did think I got air out of the driver's side caliper, but that seemed to be from loose bleeder threads. I wrapped them with teflon tape and no further air came out. But I did notice the brake pedal felt firmer the first time I pressed it, but then subsequnet applications were the same as before. I really think replacing or rebuilding both cylinders is my next best step. A smaller Front Master is an option, however doing the math (for a 3/4"), almost half the stroke would be used up just geetting the pads tight against the caliper. Doesn't leave all that much left over in case of system degradation.
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #9 ·
I have Hawk HPS pads on the front and EBC Yellowstuff pads on the rear (they don't make Hawk pads to fit the rear).

Anopther update: I forgot about my G-Tech meter, so I went for a test drive today. All I could achieve was 0.64 Gs, which works out to 187 ft to stop from 60 mph.

The car manufacturer states they got 135 feet from 60 mph. That works out to about 0.89 Gs based on 2800 lbs for car, driver and fuel.

As an aside, I also tested my RAV4 and only got 0.67 Gs.

As mentioned before, I intend to replace/rebuild both master cylinders. I didn't think they showed any sign of internal leakage, but now I see there might be a little. Stand on the brakes long enough and the pedal does depress ever so slowly. So slowly I've never noticed it driving or doing pressure tests. So slowly it shouldn't stop peak pressure from building, but it would bleed down over a significant amount of time. But I'll deal with them soon.
 

·
Race it, Don't rice it!
Joined
·
9,686 Posts
MC sq area's are:
Front 2.93"
Rear 3.38"

Calipers sq area
Front 6.77
Rear 8.97

Percentages
Front 43%
Rear 37%

That's a 6 percent built in front bias already so the bias bar should be around one turn rear. Personally I'd leave it in the middle.

I don't know pad size of the Jag stuff.


I suspect import is correct. Pads may be done for.
Hawk makes a good street pad for cold temps and easy on rotors. PFC's are the best but I don't know they have a street pad either or one that works well cold.
I also don't know where your getting 500psi at, at the pad or at the bleeder?
Something isn't right. 200psi should stop the car, 500psi should lock them up or damn close anyway.

What tire?
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #11 ·
TY for your reply. I've done some calculations of my own too. The 500 psi was measured at a front caliper bleeder screw and was the maximum I could achieve despite the pedal force calculations saying I should get more. Hence my belief that master cylinders (at least the front one) have internal leakage.

However, based on the 500 psi, which correlates to 681 psi for the rear brakes based on the diffent diameter cylinders, I calculated out 0.62 Gs which compares nicely to my measured Gs of .64.

The aim is for 0.9 Gs, which is where the tires (BFG T/As) slide on the skidpad, so I assume a skid from braking isn't far off that. To achieve 0.9 Gs, 805 psi is required for the front brake cylinder. Bsed on the pedal ratio this should be 107 lbs at the pedal.

By the way, if I calculate the percentage braking, it comes out to exactly 70% from the front brakes and 30% from the rear which I assume was a design intention.

All makes sense to me, except I can only get 500 psi at the front no matter how hard I push, and if I push hard enough I can feel the pedal ever so slowly drop.
I've order rebuild kits for both cylinders, but they won't get here for a month.

ps: This has been my first experience using a brake line pressure guage, so I had to start with analysing the design to make sense of the reading. What seems to be revealed is I have a fault somewhere and not necessarily a design flaw.
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #12 ·
I hate it when this happens, but reviewing my calculatons I found I made a mistake. I thought it was OK as the G force calculated matched up well with the value from the meter while testing the car. But no, there was a math mistake. In fact, the 500 psi front and 681 rear (100 lbs pedal effort) should result in 0.84 Gs. As in-car testing only achieved 0.64, it's starting to look like a number of you fellows were correct in thinking it may be a pad/rotor issue.

I did look at the fronts a few days ago and the pads don't have any appreciable wear and the rotor faces although not shiny smooth, still look better than resurfaced rotors. I havn't checked the rear brakes yet as they are difficult to get to (inboard brakes) but I'll get to it. I doubt those pads are worn much as they were only installed a few hundred miles ago.

I wonder how perfect eveything has to be to get maximum braking. For a power-assisted car, one probably wouldn't notice needing a little extra pressure for non-perfect brake components. No power assist for this car though.

Anyway, I apologize for erroneous information.
 

·
Race it, Don't rice it!
Joined
·
9,686 Posts
In my opinion here, aligning with import, you need bigger rotors or more pad surface on the rotor.
A quick adjustment would a more aggressive pad compound BUT rotor life will suffer. I often use up 2 sets of pads and rotors in a season.
500psi at the bleeder shows you line pressure is good. The sinking pedal maybe not so much.
I've tried the rebuild kits before but I don't generally because I find once the bpre gets scored or worn, they don't work the same.
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #14 ·
A little update. I decided to bleed the brakes once again and this time I removed the pads and re-surfaced them with sandpaper. I also scuffed up the rotors a little bit. A good improvement after bedding in the pads again. The nose of the car would now squat down significantly under heavy braking. The 0.64 G's I previously had mentioned was a one time thing. The repeatable number was 0.61 G's. Now I get 0.67 G's and it is repeatable.

Still curious, I mounted a tape measure under the car to measure the front master cyllinder stroke and found that it was bottoming out. I bought a Tilton rebuild kit and changed it out. The old piston and seal showed no signs of wear and the cylinder bore was immaculate. No difference after doing this.

Thinking something was yeilding anyway, I installed SS teflon lines. No difference after that. So it's air. Now I've bled the brakes both with the vacuum method and reverse bleeding. The last time, I removed the calipers and held them up so the flex line was vertical and tapped the caliper to loosen any air bubbles. The net result was 550 psi where before it was a little over 500, maybe even 540. Hard to be acccurate with such a small guage.

So, there's one fact I didn't ever mention, and now it's time to fess up. I'm using DOT 5 fluid which is known to hold air and can make brake bleedng difficult. Mind you the rear brakes seem to work OK. The rear tires actually locked up during one test as I hit a few tiny pebbles on the asphalt and I could hear the tires skipping on them. The rear brake cylinder only moves a small amount when activated, so the DOT 5 is working well with them.

Also, it should be noted that the front master cylinder seals showed no signs of being affected by 25 years bathing in silicone brake fluid. They were the same softness as the new seals and did not show wear. Neither the piston nor bore showed any signs of wear either; (only 9000 miles on the car, so not a lot of braking).

I am at a loss. I guess I could swap out the whole master cylinder, but I doubt very much anything will change. I could swap out the DOT 5 to DOT 4 or something. I hope I don't have to do that. I also bought some Yellowstuff front pads to obtain the best braking I can get with what I have, but until I can figure out how to keep the master cylinder from bottoming out, it makes me uneasy to rely on those pads.
 

·
Race it, Don't rice it!
Joined
·
9,686 Posts
Remove the caliper and stick a piece of wood in there and see what pressure you get. If they change, you have a calipers brackets flexing and moving around.
Another thing you might do is take the bias adjuster apart and look it over, clean, lube, adjust, ensuring the threaded rods from the heims to the MC piston are correct lengths and the recovery hole is uncovered in the bottom of the MC.
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #16 ·
T-Y for your reply. The recovery hole is OK and so is the bias adjuster. However, your thought on putting a piece of wood is a good idea and made me think of another benefit. If there's some air trapped around the piston, as it's almost all the way in due to little pad wear, I could put a piece of wood in there that allows the caliper piston to move out quite a bit. That might release some air and then I could try bleeding it again.
 

·
Race it, Don't rice it!
Joined
·
9,686 Posts
If the piston is all the way out, or close to, even with some pad wear, then a pad spacer can be used to help that. It also helps stabilize the caliper.
It happens most of time on our race cars where we are using .810 thick or even thinner rotors on caliper meant for 1.250 thick rotors.
Spacers can be bought commercially, or made on a lathe/mill, however I make mine from old pads. If they are bonded, heat the back side till the pad material falls off. Let cool and install. If they are riveted, drill out the rivets first, then heat till the pad falls off. I preferr this method to spread the loads out across the whole backing plate. We get better wear that way. Our driver can use up a set of pads in a single night so we watch them carefully.
Even a parts house discount pad is most likely a little less than the pad spacers, plus the shipping.
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #18 ·
Well... before I start I must admit to a lot of confusion with trying to diagnose my brake issue.... and so the saga continues.

I tried my method of using a piece of wood to hold the brake pedal slightly on, blocking the compensation/fill port and then using my Mityvac applied a vacuum to the caliper. I took the calipers off the steering knuckles and manipulated them around so the enlarging air bubbles could rise. I also tapped the calipers with a hammer to loosen bubbles. I also did something else I'll get to shortly.

So it worked. I got more air out, but not very much. Mind you, it did make a difference. The pedal felt firm right away.... like it should.

So now the "something else". One thing I did to help release trapped air, was to remove the calipers and pump the pedal a bit to get the caliper pistons to come out a bit hoping this might help release trapped bubbles between the bore and the piston.

And here's the rub.... I used my caliper piston resetting tool to push the piston and back in and it seemed to take more force than I thought it should although it was going back in. Then I remembered I had left that piece of wood between the pedal and the seat. I removed it and the piston went back easily.

Hmmm.... now why did the piston go back at all if the port was closed off? It must be fluid seeping past the MC seal... but I just put a new seal in it....

So another test. I monitored the reservoir level and pushed on the brake pedal. Sure enough, the level stayed constant until the pedal started to sink (at max pressure) and then the fluid started getting past the seal and caused the reservoir level to rise. THe level dropped back to normal when I released the pedal. This confirms the seal is passing fluid. Maybe I should have listened:

I've tried the rebuild kits before but I don't generally because I find once the bore gets scored or worn, they don't work the same.
Despite the bore looking perfect (although I didn't measure it) and all new guts installed, something is still wrong, so I ordered a whole new MC. I hope this will resolve the issue, but I have doubts. With only 9000 miles driven and the old piston and seal looking perfect, what would a new MC do for me... unless the original MC had a manufacturing defect (or some sort of non-visible wear)?

Once I replace the entire MC and find I still have the same problem, all that's left is the fluid (maybe because viscosity of DOT 5 is lower than 3 or 4 although it's the same as 5.1), which is the last thing I want to change (imagine the work needed to deal with the rear Jag inboard brakes... no access).

It will probably take some time to get this done. Summit was out of stock on the MC so I had to go with E-bay. I expect at least 2 weeks delivery time.
 

·
Race it, Don't rice it!
Joined
·
9,686 Posts
The port is uncovered in the first 1/8 or so of piston movement in the MC. It's there to refill the lines and caliper to compensate for wear. It should be harder to move the caliper piston back against the piston. Once the compensation port was open, the fluid had somewhere to go, back into the reservoir.

If fluid was bypassing the piston, I'd half expect to see some fluid behind the piston, inside the boot where the pedal rod goes through. Something else you can do, is plug the port at the MC with a pipe plug. The pedal should be rock hard, and stay that way under pressure.
I'd do both, front and rear MC for testing and leave the plug in to isolate the front/rear from the testing/bleeding.
If you using bias gauges, bleed those too.

I don't care for those vacuum bleeders because they tend to pull air through the bleeders screws unless you have the type that have an o-ring and thread in.
What I do, I bought an extra cap, drilled a hole for a 1/4" air coupler, connect the compressor hose to that ans put 30psi to it. Open the bleeders for 5 seconds and close it. I can bleed all four wheels by myself in 3 minutes. Use a 1/4 rubber hose over the bleeder and it's a no mess situation to boot.
 

·
Registered
Joined
·
456 Posts
Discussion Starter · #20 ·
My new MC showed up yeterday and today I managed to install it. I can now manage 700 psi (plus the width of the needle) vs. 550 psi; so just over 700psi. I might be able to get more, but I need to adjust the seat as my leg is too straight (I had moved the seat back to accomodate the piece of wood I was using to hold the brake pedal down a little bit). Didn't have time for a test drive yet, but wether the brakes can lock the wheels or not, I am very optimistic about getting at least 0.8 Gs while braking hard.

When I look back, hindsight tells me I made too much of this, however there were a lot of factors leading to confusion. First of all, my measured brake line pressure of 550 psi should have yielded over 0.8 Gs, but I only got 0.67. The fact that the front MC would slowly drop was not very noticeable and I didn't realize this was happening for a long time. Nor did I realise the front MC eventually bottomed out disallowing any more pressure increase. Some factors were sort of unkown such as the pad material co-efficient of friction and the pedal to MC geometry and what the brake line pressure normally should be. Additionally the use of DOT 5 came into question as the condition of the seals was unknown. The MC bore wasn't thought to be worn with so little use, but it turns out there was damage.

However presistence seems to have paid off. I'll know for sure after a test drive. I need to do a final bleed first (am leaving it overnight to help air release from the DOT5). I also still intend to change the front HPS pads for Yellowstuff pads, but that can wait until Spring.

Now, as for the old MC bore, I originally stated the bore looked fine. I checked it more thoroughly today. I measured it and there was no wear at all based on the internal diameter measured at locations 90deg apart. There were however some extremely light scoring as per the picture below:



These little scratches don't look like they were made by the piston. My best guess is swarf or grit left in during manufacturing. Or possibly a bit of dirt got in while adding fluid. Of course they may be other damage I couldn't find with the boroscope.
 
1 - 20 of 29 Posts
Top