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Internal master cylnder residual pressure valve

  
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Internal master cylnder residual pressure valve

 
adkalex13 adkalex13
New User | Posts: 3 | Joined: 10/13
Posted: 10/07/13
05:39 PM

Hello,
New to the forum. I have been having a brake problem with my 68 firebird and have not found a solution.

I replaced the whole system (lines, wheel cylinders, calipers, metering and proportioning valves, master cylinder).

They have been properly bled and I have no leaks. They are still spongy. I read that behind the rear cup of the M/C there should be a small spring and valve (residual pressure valve) to hold 10psi to the rear drum brakes. The M/C I bought was for disc/drum, but had no valve. I went and looked at 2 different M/C's at the auto parts store and neither one had the valve even though they were for disc/drum setup. The valve is included in a M/C rebuild kit for my car.

Why would this valve not be in the M/C that has been re-manufactured? Am I missing something? Should it be there?

My next move was going to be buy the kit for $30 for a $2 part and install it and see if it works, but I would love to have some advice as to weather or not that is my problem before I waste my time and money. Confused  

wayne712222 wayne712222
User | Posts: 94 | Joined: 10/13
Posted: 10/07/13
11:10 PM

just curious...

did you machine the drums.. since it seems to be a drum brake all the way around car..???

have you driven it to break in the friction surface to eliminate any soft surface that will cause a spongy pedal..

how is the brake pedal.. is it near the top????? climb under the car.. take a flash light and an adjusting tool or bend end screw driver...

look thru the adjusting slot to see if you can see the star wheel on the adjuster..

do not rotate it...  push it toward the front of the car.. (_____|__) then push it toward the back.. (____|_____) if you get more than 1/8 of an inch of movement. you will need to examine the adjustment of that wheel..

what.. if you get more than (___||______) that much.. the adjustment is too loose..

this is what you would find it is was loose..(___|_|___)  can you visualize what i am saying..



Wornbrakedrums Zps1038d8de  

wayne712222 wayne712222
User | Posts: 94 | Joined: 10/13
Posted: 10/07/13
11:11 PM

just curious...

did you machine the drums.. since it seems to be a drum brake all the way around car..???

have you driven it to break in the friction surface to eliminate any soft surface that will cause a spongy pedal..

how is the brake pedal.. is it near the top????? climb under the car.. take a flash light and an adjusting tool or bend end screw driver...

look thru the adjusting slot to see if you can see the star wheel on the adjuster..

do not rotate it...  push it toward the front of the car.. (_____|__) then push it toward the back.. (____|_____) if you get more than 1/8 of an inch of movement. you will need to examine the adjustment of that wheel..

what.. if you get more than (___||______) that much.. the adjustment is too loose..

this is what you would find it is was loose..(___|_|___)  can you visualize what i am saying..



Wornbrakedrums Zps1038d8de  

adkalex13 adkalex13
New User | Posts: 3 | Joined: 10/13
Posted: 10/08/13
05:12 PM

Thank you for the response.
The front brakes are disc.
The drums have not been machined.
I have put hundreds of miles on the car since doing the brakes.
I will have to check the star wheel for looseness you mentioned.
The brake feel great and are very firm at the top when bleeding. Then after a few minutes they get very spongy and firm near the floor. when pumped the pedal gets very firm again and stays that way if you keep your foot on it. That is what led me to thinking the rear wheel cylinders were retracting and I was having to pump the brakes to get them to  make contact.  

wayne712222 wayne712222
User | Posts: 94 | Joined: 10/13
Posted: 10/08/13
06:17 PM

68 firebirds have 4 piston calipers.. or did you update it to the single piston calipers

the 4 piston caliper has seals on the piston and can leak air into them as the master cylinder pressure is released when the pistons are partially seized in the bores or the bores are corroded..
there are sleeved replacements available..  

for the rears..

have the rear drums machined or replace them..

please check that the 6 wear pads on the backing plate are not worn ..

if you replace the drums or have them machined.. replace the shoes at the same time..

verify that the self adjusters actually work properly.. and are lubed with white lithium grease... NOT DIELECTRIC tune up grease..

dielectric tune up grease has dissolved sand in it.. it comes out of suspension when heated and will lock up threaded devices..  

please... also grab the axle shaft with the brake drum off..  push and pull.. lift and lower the end of the axle in the housing.. make sure there is no play that would allow the drum to move off center when the weight of the car is pushing down..

more to come...

disc brakes have the same tapered wear issues... as new or newly turned rotors have parallel wear faces..  the brake pads are parallel.. the calipers are designed for parallel operation..   if your rotors are worn at a taper.. the pads will touch at the inside part of the wear section then something has to give..

spongy pedal will result..  

adkalex13 adkalex13
New User | Posts: 3 | Joined: 10/13
Posted: 10/09/13
10:44 AM

Thanks for the info. I will find some time to test the things you describe. Thanks again.