Question: Brake pedal loss

[Johnnyfly41]

Hey guys, new to this group, great info by the way. My 71 vette loses pedal without loss of fluid. I replaced the calipers twice, master cylinder twice with rebuilt units. I replaced the rotors, that seemed to help the most, drove all summer with less than ideal pedal, but I did not completely loose it. Flexible lines are newer, no leaks anywhere. I just bought a proportioning valve from a outfit out of Hudson IA. I will install that in the spring when I get the car back out.

I have seen posts regarding runout and such. I suspect this is the most likely culprit. I still never get what I feel is a real solid pedal, be it from manual bleed or vacuum or a combination.

Please feel free to lay some of your wisdom on me.

thank you,

John in NW Illinois

 

[JohnZ]

If it steadily loses pedal after being bled and there are no leaks, it's air in the system due to "air-pumping", which is caused by excessive rotor runout; if you have more than .004" of lateral runout in any rotor, you're going to have the problem. Your '71 never had (nor does it need) a proportioning valve.

If you stand on the brake pedal and it gradually sinks to the floor with no leaks, that's a different issue - master cylinder bypassing internally.

 

[Johnnyfly41]

Thank you John Z for the excellent reply.

I thought the block on the lines near my master cylinder was a proportioning valve ? live and learn I guess.

Loss of brakes seems most severe when just cruising down the highway. With the old rotors, I would lose the pedal in a 35 mile highway trip where I never touched the brakes. I could do my in town running for several weeks using the brakes and not lose the pedal. So, that makes the air pumping theory sound right to me.

I wonder if I should be looking at the spindles and bearings, for anything causing run out there and also have the new rotors turned by a reputable place.

The part I left out, I did a autocross drivers day at Blackhawk Farms raceway several years ago. I put hawk carbon metallic pads up front and some blue, yes blue brake fluid that the autocrossers recommended. I put the stock new pads back on shortly after track day. That started my problems. I wanted to blame the fluid, but it said it was compatible with all DOT fluids. Having changed back to DOT 4 and new (rebuilt) calipers and master cylinder, multiple bleedings and then the new rotors, I quit blaming the fluid change.

I think the pad change on old rotors could have caused a problem forcing the cups in the calipers to a different place than they had run before.

Anyway, my other theory is, after pushing the car around the track that day, I think maybe it hates me.

Sorry to be repetitious, I had found a thread relating to this once before, but could not find it again.

I thank you for your patience and excellent help. Now I have to figure out what to do with this lovely billet machined block "proportioning valve" that someone sold me.

Sincerely,
John Mitchell

 

[Vettehead Mikey]

I wonder if I should be looking at the spindles and bearings, for anything causing run out there and also have the new rotors turned by a reputable place.

Most garages, reputable or not, do not have a clue how to service 65-82 Corvette brakes-especially when it comes to turning the rotors.

First off, the rotors rarely (if ever) actually need to be turned. Corvette rotors do not become 'unflat' or warp during normal use. Unless there's deep gouges that reduce the active braking surface, don't do it.

Second, the rotors are already flat and true as manufactured- there's no need to resurface them before initial use. The need to turn them is an old myth.

The root of almost all problems like your is excess play at the wheel bearings due to wear or misadjustment, and/or replacement rotors that were installed without measuring and correcting excessive runout caused by the hub or spindle, not the rotor.

The hub and rotor were machined as a matched set at the factory and were meant to stay as a custom fit pair. A replacement rotor rarely matches up with an original hub straight out of the box.

 

[Johnnyfly41]

Thanks for that additional information.

So, do I just inquire at shops and find one that can turn the rotor and hub as a unit ?

Either way, I think I am on the way to understanding this problem.

No wonder I am having these kind of troubles given what I have just read.

It's got me wonder what is going on at those rear hubs, it might be that the brakes were just pointing me to another problem.

I just had the front hubs off this spring, there did not seem to be anything amiss, cleaned and repacked the bearings. We did not check runout though.

thanks,
John

The longer I own this car, the better I like slot cars !

 

[Vettehead Mikey]

So, do I just inquire at shops and find one that can turn the rotor and hub as a unit ?

In a word, yes. Before doing that, check out how much play you have on the rear wheel bearings and correct that if required.

 

[raybies]

Hey john
Welcome to my club. I posted an article on here about the same problem your having.
I lost alot of hair getting mine figured out being as it was my first experience with a rigid mount caliper setup. As for the master cylinder and this is just an FYI, make sure you bench bleed them first and don't overstroke it.
In short though all your problems are being caused by runout. Just put it up on stands pull all 4 wheels and follow the manuals procedure for checking rotor runout and rear axles endplay. Once you get that to within .003 to.008 and pressure bleed the system again following the bleeding order you should be good to go. Just another FYI, if you buy rebuilt calipers make sure they are stainless steel sleeved. It'll save you alot of grief down the road.

Good luck
Ray