Like the subject says, the '72 vette in my garage has been into many shops multiple times and has cost a friggin ton of money. And its STILL undrivable.

For nearly 4 years now the car has had a problem of the brake pedal going straight to the floor. Odd thing is after its been worked on and I go to pick it up, the brakes work great. After its sat 2-3-4+ days in the garage I pull it out and pedal smacks floorpan again. Every damn time!

First shop replaced the master cylinder twice.

Second shop replaced the proportioning valve (refurbished, cant get new ones anymore), new master cylinder again, and redid all the brake lines.

Third shop replaced a caliper and the master cylinder.

Last shop thats had the car since February has replaced the booster, the other 3 calipers, a few lines, everything that has the word vacumn in its name, and pressure bled several times. (All shops pressure bled btw) They didnt replace the cylinder thankfully ($475 a shot)

I got the car back delivered Thursday night, didnt get a chance to drive it but the brake pedal was nice and firm while I parked it and didnt get anywhere near the floor. (Car was driven 30 minutes to my house, worked great) They had it finished since Tuesday and had been driving it since then fine.

Yesterday I pulled the car out to take it for a spin, back out and 'thunk' pedal smacks floor again.

This cars old, but everything thats got anything to do with brakes has been replaced. No-one can figure it out. It goes back to the last shop tomorrow since they warranty work and since the car isnt working they will work on it free of charge till it does. (tho they aint having it another 8 months if I can help it)

Does anyone have any ideas??? Is there something that I or any mechanic thats looked at it could be overlooking??

I have an assembly manual comming in the mail, Im gonna check the part number for the master cylinder to make sure it is the right one. Thats all I can think of, that its the wrong one.

Anyone??

 

hey that IS a thirdgen question, just not a thirdgen f-body question

are you leaking fluid anywhere?
is air leaking into the system anywhere?

 

Do you have to pump the brake after it hits the floor to make return to normal, or is it dead - period?

I would look at something they may of missed, like the mechanical linkage from the brake Pedal to the booster.

Could be just crappy master cylinders. I seen a guy with a 87 Vette pull his hair out with a alternator before. It turned that he was buying the same brand, and I guess there was a bad batch on the shelves.

I also went through this with spark plugs once. Had a miss - replaced spark plugs with new one & moved on. Couldn't get rid of the miss. Replace wires - bla bla bla, still there. Took it to a dyno, and sure enough, I replaced a bad set of plugs with another. It was sucky luck, I guess.

Ron

 

If you're not leaking anywhere externally, about all that's left is the master cylinder, prop valve, and calipers backing off.

Ron's question could be pivotal - can you "pump up" the brakes with the pedal?

 

The reason I'm concerned about the ability to pump up the brakes is that it is possible you have caliper issues. I understand that one was replaced, but there are three remaining. If one of them has a broken, weak, or incorreectly installed piston spring (even the replacement one), you might have a caliper backing off while the car is sitting idle. Until the fluid is pumped back into the caliper, there would be excessive pedal travel. I'm basing all this on the fact that you car should have the older four piston Delco Moraine calipers at all four wheels.

 

he first said one, and then said other 3, so he got em all

 

Yeah, I covered my butt on that one, didn't I?

The reason I mentioned the replaced caliper is that I have received remanufactured parts that were incorrectly reassembled in the past. Everyone has a bad day now and again, even the rebuilders.

Not out of the realm of acceptable diagnostic procedures would be the possibility of capping or plugging the brake lines at either the front or rear end of the proportioning valve to help isolate the problem. The same could be done at each caliper to isolate it further.

 

i apologize vader, i guess i didn't read you post close enough, and i know what you mean about "new" parts, that are junk from day one

 

Thanks for the replies.

No, no amount of pumping gets the brakes back to as they should be. While the last shop was working on it they did have that kinda situation where the first 3-4 pumps would floor but it would be fine after that. That was before they replaced the booster and it worked for them fine after that.

Interesting point bout the calipers Vader, we will definately try and cap off each one at a time and see what happens.

Ill also check the actual brake pedal and all that as well. I wouldnt have thought that could decide to work sometimes and not others, but Ill try anything at this point.

The shop owner has a feeling that air is somehow seeping through the master cylinder when the car sits after its just been bled and all that good stuff. Its gonna be replaced yet again.

Thanks again. Update will be a few days, my guy will finish my new caamro engine first, then work on the vette brakes.

 

Just a side note:

My neighbor had an odd brake problem with his '82. A few shops looked at the problem, replaced lots of parts, and still no solution. They finally replaced the hoses at the rear split axle (ALL of them) and the air bleed problem was solved. I'm not saying you have a hose problem, but apparently it isn't uncommon to have air entrapment issues on the thirdgen 'Y' bodies.

 

I know this dosen't help but for all that money you could have bought a complete wilwood brake system...

 

Master cylinder... new or reman? If its remanufactured, just take the one thats in there, toss it, and get a NEW one. Remans are junk IMO, they were nothin but problems when I used to sell parts.

As for the real problem (if its not the master) you should check/replace all the lines in the car. Both rubber and steel. I had a problem on a car of mine, took a bit of searching to find it too. There was a pinhole in one of the brake lines, the metal ones. It was spraying out fluid when I put the pedal down, causing a soft pedal. I never saw any fluid leaks, mainly because it was spraying fluid right onto the headers, and vaporizing. Cute huh? I thought so. Anyway, I replaced that line and the brakes have been great since.

I have heard of master cylinders leaking out the back side and into a power booster, which not only hides the fluid leak, pressure leak, but ruins the power booster as well. Just a thought.

 

FORMULA355TPI, car is 100% origional.

The current master is rebuilt I believe, but is about to be replaced again anyways.

All lines have been redone, steel and rubber. But we will have them looked over very thouroughly for pinhole leaks. didnt expect to find something soft like that. Though there is no fluid on the floor, or on the car, and there is no evidence of loss in the master cylinder as the fluid level stays the same. Plus like you said madmax the pedal would be soft and spongy,... this pedal smacks floor with almost zero resistance.

Does vacumn effect brakes on cars this old? I dont know a whole lot about it yet.

 

Hi,

Many times brake problems on these cars 1963 thru 1982 Corvettes can be traced to the rear discs. The factory trues the rear rotors and spindles as a riveted assembly. Most of these cars have had the rivets drilled to do emergency brake work and/or spindle bearing replacement from lack of greasing service.

The rotors get arbitrairily (SP?) put back on the car. They can and do often have enough runout to pump air past the seals on the calipers. Often can be cured by rotating the rear discs in relation to the spindle to correct the runout oscillations.

The rotor thing mentioned above doesn't seem to be commonly enough known and the brake guys can never figure it out because they are not Corvette guys.

 

Sounds completely plausible to me. I learned something else again today. I'm filing that one in the "old elephant" banks right now, since I know it's going to come up again somewhere.