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Power brakes dont feel right...any suggestions?

13K views 80 replies 11 participants last post by  chevyrestore 
#1 ·
Hey guys. Got a 59 chevy truck with the classic performance power brake conversion on the frame (not up on the firewall).

My situation is the brakes work great but they always feel too hard, best way to describe it is when I hit the brake pedal the truck slows down but much slower than it should and the pedal is rock hard.

If I do a panic stop I can NEVER lock up the wheels, sometimes under a panic stop the pedal goes down just more than 1/2 way and is rock hard but the truck keeps going.

Is this a problem with the type of pad or a lack of vacuum or pushrod adjustment or what?

15-18" vacuum at idle, autozone semi-metallic pads, new brake fluid. Running the classic performance proportioning valve as well.

Thanks for any advise.
 
#31 ·
Hers's the deal...Typically a kit purchased for a disc/drum combo will come with a 1 1/8" master cylinder as you don't need as much line pressure with that combo.

If the kit came with a combination valve it will be incorrect with a disc/disc system because of the 10lb residual pressure valve going to the rear brakes and the metering portion will be wrong. It sounds like you had a stand-alone residual pressure valve, and in that case you should have switched the 10lb for a 2lb. The 2lb check valve keeps the pads from backing away from the rotor when you are not pressing the brake pedal. Also make sure they are installed in the proper direction.

With your booster, a 1" bore master cylinder with a 2lb check valve in the line to the front brakes and another in the line to the rear brakes along with an adjustable proportioning valve, (or the proper combination valve) will give you good brake action. I will assume that you haven' modified the brake pedal assembly as it came from the manufacturer.

Does the master cylinder you are using have a deep (about 1") depression in the rear of the piston, or a shallow one (about 1/4")? In most cases you need the shallow one with a power brake booster. Again, a photo showing the unit and the piston would be most helpful.

Lastly, what type of calipers are you using on the rear?

If we beat on this long enough it will eventually give in and work for you!

Andy
 
#32 ·
That is a very common master cylinder. They came installed on 1982 Corvettes if I remember correctly. They are available in 1 1/8" and 1" bores, but I don't think they came in 1 1/16".

With the master cylinder bolted up to the booster, how far can you pust the rod on the pedal side of the booster before you feel it compressing the master cylinder piston?
 
#34 ·






The nut I am talking about is the small 8mm or so head on the booster that the master cylinder pushrod sits on. I cant push the pedal and feel the master cylinder at the same time so ill have to wait on that one until I get a helper. Honestly I feel like the booster is already touching the pushrod in the master when its bolted up.

As for the valves all I have is the hard brake lines going into the correct prop valve as per CPP's hookup diagram. I verified that the prop valve is for a disc disc setup, they did not say to use any additional valves. Not sure what you are talking about as far as the depression in the MC but I uploaded a few pics here.
 
#36 ·
Interesting...

What it looks like you have is a manual brake master cylinder with an insert installed to take up the space to the booster pushrod. Not a problem, but I haven't seen that before.

You also have a combination valve, and if I understand you correctly that is the same one that you used when it was disc/drum correct? If so, that is the wrong unit for disc/disc. Did you remove the drum brake check valve out of the combination valve, out of the master cylinder (some masters have them built in) or was it an inline valve?

What I would do is this. Disconnect your brake pedal from the booster so it isn't pushing against it at all. Put that spacer into the master cylinder, lengthen the pushrod coming out of the booster quite a bit. Then, slide the master onto the booster and you should feel the piston hit the pushrod. If so, shorten the booster pushrod and retest until you can just slide the master onto the booster without feeling resistance, then shorten it about 1/2 turn more. That way you can be sure the booster isn't preloading the master, and that there isn't too much clearace. Bolt on the master cylinder to the booster. Then, adjust the pedal pushrod until you get the pedal height you want and tighten the jam nut.

Now is where it gets interesting. If you like, bleed and test the system and see if it is any better. If not, remove the combination valve and set it aside. Install a 2lb check valve, one in the front lines near the master cylinder, and one in the rear line along with an adjustable proportioning valve in the rear line, bleed and retest. I think then things will be much better.

I still would like to know what kind of calipers you have on the rear axle.
 
#37 ·
the rear end is from a 93 camaro with the stock calipers, just verified the pistons move freely and put on brand new pads and rotors, no change in braking.

What insert are you talking about? If its the 3rd pic on the gallery thats what CPP calls the master cylinder pushrod (between the master and booster)

When it had rear drums it did NOT have the prop valve, just a residual valve inline going to the rears, when I switched to disc all I did was remove the inline residual valve and add the disc disc prop valve, used same master cylinder as per CPP's directions (although I cant guarantee the guy that gave me the advise on reusing the master was all that bright).
 
#38 ·
That "master cylinder pushrod" is just a filler that company makes so you can use a manual master cylinder with a power brake booster. Since you can still adjust the pushrod in the booster, my technique for adjusting the clearance between the master and the booster is still valid.

Since the combination valve is for a disc/disc setup, you should be okay there. One other little quick check is to make sure there is no residual pressure check valve in the master cylinder itself. Take a small allen wrench or drill bit and put it into the fittings on the master where the brake lines attach. If there is a check valve, the tool will only go in about 1/8" past the seat that the brake line seals on. If there is no check valve, it will go in about 1/2" or a little more. You are probably okay there, it is just good to check.

Make sure that if there is a way to adjust the parking brake on those rear calipers that you adjust them up snug.
 
#39 ·
one side of the MC has 2 ports that are plugged with allen plugs, the other side has 2 ports that are occupied by the prop valve, do I need to check all 4 or only the 2 being used?

Ill adjust the booster tomorrow when its light out, if there is no internal valving like you said would this be the same as a disc disc MC or would it be beneficial to get a true disc disc MC?

The rear calipers do have an adjustment for the ebrake but its a weird setup, There is a small hydraulic piston that the brake pedal uses to push the pads into the rotor, then there is a bigger ebrake piston thats mechanical around the hydraulic piston (basically a piston inside of a piston), from what I can tell even if the ebrake was fully out of adjustment the hydraulic piston would still push out the pads and be in contact just as much as if it were adjusted.

I dont have ebrake cables connected because its a huge job to add that right now, is it possible this is a problem? I showed a GM tech that works with me and he said it looks like it wouldnt affect the hydraulic portion of the caliper and shouldnt be needed to operate the pedal properly.

I remember when putting in these brake lines that there were adapters in the MC, I tried to take those off and put the lines in by themselves and it went WAY into the cylinder so I put the adapters back, I guess that means there arent any internal valves?
 
#41 ·
You only need to check the ports that you are using. Having that check valve in the master when you don't need it won't make for hard braking, it will just make the pads drag on the rotors. It is good to check for it. If you find one, there is a way to take them out. Just let me know and I can talk you through that.

The lack of the check valve, and large reservoirs are the charactersitics of a disc/disc master cylinder, so if there are no check valves that master cylinder is fine to use.

It is possible that if the emergency brake is not adjusted properly it could reduce braking performance. I would do a GOOGLE search for those calipers and see what others have to say. The calipers on the 9" Ford rear axles out of the Lincoln Versailles are famous for that problem. Unless the parking brake is adjusted up tight, you will never have good rear brakes.

Keep us posted on your progress.

P.S. That combination valve looks pretty low to the ground. It isn't gonna drag on speed bumps or anything is it?
 
#42 ·
no on the Prop valve being close to the ground, its pretty high up but the only way I could get a good pic was in a weird angle, it looks much lower than it actually is and I have a crossmember directly under it to protect all the brake components from bumps and rocks or anything that may try to come into contact with it.
 
#44 ·
Thanks will do. I already bled the brakes properly 2 times around to verify no air (yes the correct way farthest from the master to the closest) :)

I didnt need to crack the lines to pull the master back for the pics so ill do what I can to adjust the rears manually and the adjustment of the booster and give it a shot.
 
#45 ·
What I am really interested in once it is all put together is how much fluid volume you are getting at different points in the system. Do you get a real strong shot of fluid at all the calipers when you are bleeding them? Sometimes the front will have strong volume and the rears, not so much. There are different reasons for this that we can talk about if necessary.

Good luck!
 
#46 ·
Alright got an update, I turned out the booster rod (between the booster and master) a bunch and tried to put the master on, no go, turned in the rod a little, better but still hitting the master, turned in more and it was flush but hitting the master just barely, turned it in just a hair more, master bolted up and not hitting the rod....
then adjusted the heim joint between the pedal and booster, turned it all the way in and there was no play, turned it back a little, now theres a little play, turned it just a hair more, now theres about 1/2" play (when I push the brake pedal it goes about 1/2" before I feel anything push on the MC).

Test drove it and its MUCH better but not right yet. I was able to slow down much quicker but still impossible to lock up the wheels (maybe because of the 454 motor and heavy truck with cheapo kragen pads??)

Should I adjust the booster again or look at better pads or what? I do have access to the depth gauge for GM boosters at my shop and can try that out also or is this as good as its going to get with the cheap pads, heavy truck, and massive engine??

I did notice that now under panic stops the engine stumbles a lot due to all the air being sucked up by the booster, maybe it needs some more vacuum to operate panic stops better?
 
#47 ·
Glad we are making progress...

Does the pedal "pump up" when you are pumping the pedal, or does it stay at the same height every time you push it?

If you hold firm pressure on the pedal does it sink to toward the floor?

How high above the floor is the pedal when you are pushing on it hard?

If it stays the same and doesn't sink down, then try this...

Go into a gravel parking lot and drive about 15 mph and hit the brakes hard. If any wheels lock up, you will be able to see by the ruts in the gravel which wheel or pair of wheels it was.

Let us know what you find.
 
#48 ·
The stumble is more than likely a carburation issue and not due to vacuum.

It sounds like you are right on with the master/booster adjustment. Leave it as-is.

Cheap pads usually equal soft pads. Soft pads usually stop better cold than high performance pads. Don't get me wrong, there are some cheap pads out there that don't stop for a damn.

Lets keep working with what you have before you go and spend any more money.
 
#49 ·
when pressing hard enough to just stay stopped like at a light the pedal is about 5 1/2 inches from the ground (little less than 1/2 way from the at rest position), when pressing hard like for a panic stop its about 4 1/2 inches from the ground.

When pumping the brakes at idle the pedal goes down the same amount each time, when holding it down at idle it stays in position also.

Stopping enough to thrust me forward but not enough to lock up the wheels still.

Dont know ANYWHERE that there is loose gravel around here, ill keep an eye out but most of the places with gravel are construction areas and blocked off here in the city. Dont know if ill be able to find anywhere large enough to be able to do that gravel test, just with the weight alone I think the whole truck would slide anyway.
 
#51 ·
:) That was the first thing I did when I starting dealing with this brake concern, pedal sinks, unless you want me to do it again. I have also pinched the brake hose from the intake to the booster and driven it, rock hard pedal then, I know the booster is working properly at that point.
 
#54 ·
Auto parts stores sell a special hose for brake boosters. It is described as 11/32" vacuum brake hose. It has a thicker hose wall and won't collapse internally, and being a slightly smaller diameter you don't typically need hose clamps if using it on a hose nipple fitting (though you can clamp it if you like).

For a hose run as long as yours (booster under the floor) I would recommend 3/8" hard line from the intake down along the transmission to near the booster, then a piece of vacuum brake hose from the hard line to the check valve on the booster.

At very least replace all the hose you have with the special hose.

Once all that is good, we can look at brake pressure at the wheels.
 
#55 ·
ill pick some of that up but in the meantime I have verified that this hose is NOT collapsing, in the meantime can I keep on checking the system with this hose? I just got off the phone with the parts store and they had to order it so I wont have it for a couple days.

Anything else to check also before going into specific wheel pressures?
 
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