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Topic Review (Newest First)
05-25-2009 02:22 AM
TheCiscoKid
redoing my brake setup, need advice

The calipers are 80s camaro, the pedal ratio is actually 4.85/1, MC bore is 1'', it is a chrome smooth after market type, rear axle is a 10 bolt 80s so not sure what size the cylinders are. Pedal rod travels 1'' maybe bit more if really pushed.
The brakes seem fine while driving, slow the car without much pedal force.When coming to a stop, the car doesn't want to stop fast. I just took the car out on gravel and front brakes will lock, but I have to really stand on the pedal. So what will make this better? I'm going to put residue valves in the rear lines. Can the combination valve be out of sink?
03-15-2009 06:50 PM
TheCiscoKid
redoing my brake setup, need advice

You have a set of GM calipers in the front, which ones, metric or early calipers? >>> came with the TCI kit ,so probably metric ??
What is your master cylinder bore size?>>> 1"
What is your measured pedal ratio? >> from pivot to rod 4''. Pedal length is 16'' from pivot , so 4:1 ??
How much travel at the master cylinder pushrod do you have using the full available pedal stroke...? >> rod moves about 3/4'', might move a bit more but car won't start, so much for the new Optima battery.
What size are your rear wheel cylinders? >> not sure 80's Chev 10 bolt axle.

About pedal travel, I shortened mine, starts just on high side and passes the verical.
03-12-2009 11:45 PM
aosborn A couple thoughts on this situation...

First, the only difference between a "disk" and a "drum" master cylinder generally is the reservoir size and the addition of a check valve. Disk brake master cylinders usually have large reservoirs on both ends, and do not have check valves in the outlet ports. To verify if a check valve is installed or not, remove the line from the master cylinder and gently insert a small rod into the hole in the brass fitting seat in the master cylinder. If the rod goes in over 1/4" or so, there is no check valve installed. If you hit an obstruction, that is the check valve. That would be the port for a drum brake axle with no other residual valve required. If neither of the ports have a check valve, that master cylinder can be used with drum brakes, you just need to add a 10# residual valve in-line.

Second, without knowing what type of car the "GM" combination valve was designed to be installed in, it is a crap-shoot if it will work perfectly on your car. Telling the difference between a disk/disk or disk/drum combo valve (if my memory is correct) is the installation of a check valve for the drum brake axle port (it wouldn't be in the master cylinder in this case).

Third, since your car is unique in its center of gravity location, tire size combination, etc, the possibility of installing the perfect "factory" style combo valve is slim. I would chuck the combo valve, make sure there is a 10# check valve somewhere in the drum brake circuit, and install an adjustable prop valve in the rear line. (I haven't used a stand alone metering valve in my many projects, but I would look into it for future use)(I have set up many excellent braking systems without one). Set it to full pressure and test the car in an open gravel parking lot at slow speed. After warming up the brakes a bit, jump on the pedal and see if you get any lock-up. Like was stated earlier, adjust your brakes so the fronts lock up JUST before the rears in a high speed full brake application (work up to this while adjusting your prop valve).

Lastly, make certain everything is installed, adjusted and bled properly. Especially the push rod between the master cylinder and the booster. Also check that the brake pedal linkage doesn't go over-center in its normal travel. I have seen some cars that this was the case, and when the pedal moved past the high point of the arc, the harder you press the pedal the brake pressure was actually going down!

Good luck,
Andy
03-12-2009 11:19 PM
powerrodsmike I'm sorry, I missed the part about the firewall mounted master cylinder that you put in your original post....disregard the stuff about under floor mounted masters and pedals.

it is easy to tell the difference between a combination valve for disc/disc and one for disc/drum.
The rear outlet looks much the same, but on the disc/drum combination valve between the 2 outlets for the front lines, there is a small screw in fitting with a button that sticks out, usually these are covered with a rubber boot. That is part of your metering valve. The disc/disc combination valve does not have this.
The wrong combination valve would not make your brakes feel bad, or not lock at least the fronts properly.


Residual pressure valves are needed for the rear drum brakes , those are stock in just about all applications. Having 2 valves in the line will not affect the function at all.

If the pedal feels ok, but the brakes don't lock, you probably have air in the lines, or your calipers are pulling back more than they should, or the rear brakes are not adjusted, there are a few other reasons, but start with those.... You can run a 2# RPV in the front line, it won't hurt a bit, it usually is not needed, but there are times when it will help...more on that after you answer a few questions...



Most cars in the weight range of a 38 chevy that use disc/drum use a 1 1/8" bore MC with an 11" booster and a 4:1 pedal ratio. If you lengthened a pedal to fit at the bottom, you changed the pedal ratio to a higher numerical ratio.. If you lengthened it at the top, you lowered the ratio.

Lets forget about valving for a little bit and get some of the basics out of the way.

You have a set of GM calipers in the front, which ones, metric or early calipers?
What is your master cylinder bore size?
What is your measured pedal ratio?
How much travel at the master cylinder pushrod do you have using the full available pedal stroke...?
What size are your rear wheel cylinders?

Actual measurements please..

The disc/disc master cylinder is no different from a disc/drum master cylinder other than it has no RPV installed in the rear line. Drum/drum MCs have 2 RPVs...The range of available bore sizes are the same.

Some GM cars use a step bore master cylinder, but we won't go there just yet.

later, mikey
03-12-2009 09:51 PM
TheCiscoKid
redoing my brake setup, need advice

Now I'm confused as posted last year and was told to use residue valves and an adjustable proportioning valve.

Front brakes were in the TCI IFS kit, GM disc.So no to small pistons. I firewall mounted the booster with a S10 petal lengthen about 1''.The pedal travel feels about right and it doesn't feel hard to push.Even if I do push hard the brakes don't lock.
I was wondering if I have the wrong combination valve as a disc/disc or a disc/drum GM valve look the same.How do I tell them apart?
Is my master cylinder (equal chambers) a disc/disc unit ?

Cisco
03-12-2009 04:28 PM
powerrodsmike Watch out that you don't run out of pedal travel at the pushrod if you go to a smaller master cylinder than the one you have. The underfloor pedals typically have a pedal ratio of 6 or 7 :1, and the distance they can move the pushrod is limited by the shape and size of the pedal. Disc brakes do require more volume of fluid to be delivered to the caliper, hence the larger bore and power assist requirements.

If you can get 1" of pushrod travel with your current setup before you lock the brakes, , when you go to a 7/8 master you will never lock them. If you were running a different caliper, such as a Baer or Wilwood with smaller pistons, the smaller bore would work.

I try to set the systems I build so they lock the brakes at 3/4" of pushrod travel, and the full pedal travel will move the pushrod at least an inch with the bleeders open.

I've done several swaps using the same setup you have, and the 1" and even 1 1/8" seems to work alright with the underfloor pedals, and 7" booster and even as low as 16" of vacuum, but there is some extra pedal effort. (how much extra? hard to define, but to me it was acceptable)

Don't forget, when you decelerate, your motor will develop much more vacuum that what it shows at an idle. And when you hit the brakes, (unless you are doing brake stands), you should be decelerating.


I just put an auxilary vacuum pump on a guys car with much the same setup as you, he had about 15" of vacuum, it made the brakes marginally better as far as pedal effort, but the noise it makes is undesirable...( it sounds like a cow making a moo sound.)

I have looked at those vacuum/pressure charts, and found them to be somewhat exagerrated.

If you have a disc/drum setup, you do want to use a metering valve in the front brake line, these are found in a disc/drum combination valve or you can buy them as inline stand alone units , which double as a "Y" for the front brake line.

The adjustable rear proportioning valve is just a pressure limiting device, and does not care if it is for disc or drum.

Later, mikey
03-12-2009 07:32 AM
willys36@aol.com First, don't fret about line size. All brake lines do is transfer fluid and their diameter has zero bearing on brake function. Can be 1/8" or 1/2" brakes will function the same. Critical dimensions are the master cylinder/slave cylinder ratio and booster pressure output. The dual 7" is good for ~1200psig @ 18" vac and 1" bore which apparently isn't enough for your application. According to the list below, you probably need to go to a 7/8" or even 3/4" master cylinder or to step up to a 9" or larger booster which may need to come from a Pick-A-Part car? As your system stands, you don't need the resid valve if your pedal stroke is not excessive and you don't need a proportioning valve which is only needed if the system is unbalanced, especially if the rear brakes lock up before the front. That is a dangerous situation causing your car to endo faster than Hussein can sign a $trillion spending bill or Slick Willie could undress a White House aide. Your's doesn't sound imbalanced - it is uniformly bad!!

Here is some info I posted in another thread;

"The 7" street rod booster requires 18" of vacuum to operate properly. With this vacuum level the booster will produce a maximum pressure output of 950 psi with a 1" bore master cylinder. This pressure level will work great for four wheel drum systems and disc/drum vehicles which are relatively light. This booster is not recommended for heavy disc brake cars like 55 Chevy's or for any four wheel disc brake system."

"The 7" dual diaphragm street rod booster requires 18" of vacuum to operate properly. With this vacuum level the booster will produce a maximum pressure output of about 1200 psi with a 1" bore master cylinder. This pressure level will work great for four wheel disc systems and disc/drum vehicles which are heavier."

"The 8" booster requires 18" of vacuum to operate properly. With this vacuum level the booster will produce a maximum pressure output of 1200 psi with a 1" bore master cylinder. This pressure level will work great for four wheel disc systems and disc/drum vehicles which are heavier. This booster is recommended for heavy disc brake cars like 55 Chevy's or for any four wheel disc brake system."

"The 9" dual diaphragm ultra booster requires 18" of vacuum to operate properly. With this vacuum level the booster will produce a maximum pressure output of 1400 psi with a 1" bore master cylinder. This pressure level will work great for four wheel disc systems on heavier cars and trucks."
03-11-2009 10:24 PM
TheCiscoKid
redoing my brake setup, need advice

My garage will soon be unfrozen, guess it's time to get back out there. The brakes on my 38 Chev weren't the best, seemed fine driving but not stopping, won't wheel lock.This is a GM disc/ drum setup. Motor pulls 18hg at idle.
I firewall mounted a 7'' dual diaphram stainless booster, a smooth chrome master cylinder, 1'' bore.The MC has equal size chambers and uses 3/16 line.>> >is this a disc/ disc MC? and useable?. I had used a GM combination valve.
I was going to put 10lb residue valves in the rear lines and got a 'Stainless Brake' adjustable proportioning valve for the rear. It was more money but I liked that it also provided a mount for the front lines and has a brake switch and it was CHROME. It only takes 3/16 line and I asked the company and they say its for disc brakes , but could work???. I don't think their literature addresses use for disc or drum brakes. >> think I should use it? or get another proportioning valve?
thanks Cisco, and it's still fricking freezing out

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