My own strange brake problem....

[74bigblock]

So last summer I replaced with new the following:

Booster
Master
Lines to Proportioning valve
Lines from valve to Left & Right Front Calipers
Both Front calipers
Front rotors.

They haven't been the same since, and I think it's either a bad master or the proportioning valve. I just had my local shop pressure bleed the entire system 3 times to ensure that all the fluid was fresh, and there was no air in the lines. With that said... here's what is still happening:

Symptoms:

With car off, pump up pedal... good for about 3 pumps... then it goes soft. It does not come back until I start the car, then turn it off again.

With the car running, the brakes function, but do not have great stopping power, and I'm getting a ton of squeal out of the front.

Any thoughts??

Thanks,
Dave

 

[74bigblock]

BUMP! ;help

 

[wishuwerehere82]

Hi Dave!

I will take a crack at it.

The soft pedal would indicate either an air bubble in the lines, or a bad Master Cylinder. You should do some tests with the engine running
1st push the pedal down and hold the brakes on. Push it hard. Is there any spongey feeling in the pedal? This would indicate air in the lines or in the calipers.
I think we have all been through this at least once or twice. You need to bleed the system after driving it to completely get the air bubbles to the top of the calipers and out of the pistons. They like to hide in the lower areas of the calipers, and a good shaking will get them up to the bleeders.

2nd Hold the pedal down and then release the pressure so you are just barely holding the brakes on. Does the pedal start sinking to the floor? This would indicate the master cylinder is passing fluid by the O-rings in the cylinder and is not a good one. Some new parts still have been known to be bad.

As for the front brake squeel, did you put on new pads with the rotors, or use the old ones? This is what usually happens with using the old pads with a new rotor. You could try scuffing the pads and crosschecking the rotors with 240 grit wetordry sandpaper to resurface them and see if it goes away.

 

[gmjunkie]

Hi Dave!

I will take a crack at it.

The soft pedal would indicate either an air bubble in the lines, or a bad Master Cylinder. You should do some tests with the engine running
1st push the pedal down and hold the brakes on. Push it hard. Is there any spongey feeling in the pedal? This would indicate air in the lines or in the calipers.

2nd Hold the pedal down and then release the pressure so you are just barely holding the brakes on. Does the pedal start sinking to the floor? This would indicate the master cylinder is passing fluid by the O-rings in the cylinder and is not a good one. Some new parts still have been known to be bad.

As for the front brake squeel, did you put on new pads with the rotors, or use the old ones? This is what usually happens with using the old pads with a new rotor. You could try scuffing the pads and crosschecking the rotors with sandpaper to resurface them and see if it goes away.

I agree!!!

 

[wishuwerehere82]

I agree!!!

Hey Junk!
There you are! I was waiting to see if you would see this and tackle it first.

 

[gmjunkie]

Hey Junk!
There you are! I was waiting to see if you would see this and tackle it first.

I just seen it today!!! I've been working on Medusa and not paying enough attention I guess!!! Medusa is alive and well now,Damn is she hard to hold back!!!cool:cool

 

[74bigblock]

Hi Dave!

I will take a crack at it.

The soft pedal would indicate either an air bubble in the lines, or a bad Master Cylinder. You should do some tests with the engine running
1st push the pedal down and hold the brakes on. Push it hard. Is there any spongey feeling in the pedal? This would indicate air in the lines or in the calipers.
I think we have all been through this at least once or twice. You need to bleed the system after driving it to completely get the air bubbles to the top of the calipers and out of the pistons. They like to hide in the lower areas of the calipers, and a good shaking will get them up to the bleeders.

2nd Hold the pedal down and then release the pressure so you are just barely holding the brakes on. Does the pedal start sinking to the floor? This would indicate the master cylinder is passing fluid by the O-rings in the cylinder and is not a good one. Some new parts still have been known to be bad.

As for the front brake squeel, did you put on new pads with the rotors, or use the old ones? This is what usually happens with using the old pads with a new rotor. You could try scuffing the pads and crosschecking the rotors with 240 grit wetordry sandpaper to resurface them and see if it goes away.

Humpphh :cry

I did replace the pads... twice (thinking it was the pads).

I'll fire her up when I get home tonight for the master test you mention above.
I can tell you it's spongy at the bottom with weight on the pedal. I can feel the pedal hit the stop.

 

[gmjunkie]

Humpphh :cry

I did replace the pads... twice (thinking it was the pads).

I'll fire her up when I get home tonight for the master test you mention above.
I can tell you it's spongy at the bottom with weight on the pedal. I can feel the pedal hit the stop.

I'll bet the master is bad!!

 

[wishuwerehere82]

Humpphh :cry

I did replace the pads... twice (thinking it was the pads).

I'll fire her up when I get home tonight for the master test you mention above.
I can tell you it's spongy at the bottom with weight on the pedal. I can feel the pedal hit the stop.

If the pedal is to the stop, that is one way to cut up the O-rings in the master cylinder. I bet that is your problem too.

 

[74bigblock]

Yeah so it seems like it's just air in the lines or in a caliper. It's just plain spongy.

 

[wishuwerehere82]

So start bleedin them. Find a helper to step on the pedal and git'er done!
Don't forget the back ones have two bleeders on each caliper, and you will have to pull the wheels to get to the outer ones.
Do right rear first inner and outer, left rear inner and outer, right front, left front.

 

[bossvette]

try one of these on for size
http://www.motiveproducts.com/02bleeders.html

 

[zachh]

Proportioning valve will do it. My Ttans Am has the same problem. The piston in the prop valve might not be seating fully or correctly auseing your problem. The BEST way to bleed brakes is with two people, one loosening the screw and one pumping the pedal. I use pressure bleeders all the time at my job and they dont do nearly as good as a job as the 2 person way. Sometimes that is what shifts the prop piston causeing problems. Ive seen it happen many times, and it aint pretty. A bad master cylinder might also cause your problems. I would also check your booster for eaither a vacuum leak or bent push rod. If the vacuum cannot escape the diaphram, it "over boosts" in a way.
best wishes
zachh

 

[L48]

I used that power bleeder on my recent brake work and it worked well. The pressure you use affects how well it bleeds. 12 psi worked a lot better than 6 psi at pushing the air out. A couple pumps of the brake with the pressure off mid-way though the bleed may have also helped move the air bubbles to where they could be pushed out.

However, I had to redo a leaky caliper and when I re-bled it I used a hose and a medical syringe (well the plastic part not the needle part) to suck the air out through the bleeder.

 

[JohnZ]

If the pedal sinks gradually to the floor, the master cylinder is bypassing internally. '74 Corvettes didn't have a proportioning valve.

 

[bossvette]

'74 Corvettes didn't have a proportioning valve.

I'm curious, my 68 has what I assumed was a proportioning valve (a brass block the lines connect to) as do my other C3's what purpose does the block serve?

 

[wishuwerehere82]

I'm curious, my 68 has what I assumed was a proportioning valve (a brass block the lines connect to) as do my other C3's what purpose does the block serve?

As you know, the block has 2 lines going into it from the master cylinder.
One line is for the front brakes, and the other is for the rears.
There is a spring loaded piston inside the block that is connected to a switch.
If for any reason there is more flow in either the front or rear brake fluid, which would indicate a blown line or seal in the brake system, the piston will be pushed to either end of the block and turn on the brake warning light (idiot light) on the dashboard.

As if you didn't notice the loss of brakes already!

 

[Evolution1980]

With a new master cylinder, I'd just skip the proportioning valve altogether. From what I can recall, it's really not all that necessary.

You didn't mention anything about your rears. Did you check your rear lines and calipers?

Here is the proper way to bleed the system.