rear brakes locked up solid - help

[Gavin]

Run AP Racing calipers Front, Corvette Calipers on rear - three + years of track events and have proven very effective and reliable.

Outsmarted myself by experimenting with a different and thicker rear pad. Big mistake - Ran Saturday at Button Willow without really getting on it at temps were chilly - I got a lot of rear lockup.
Sunday it was warmer and we were able to get heat in the tires and run harder - lost pedal pressure a few laps into a session - pitted to find the rear rotors were extremely hot, considerably more heat coming off them than off the fronts.
Seams to me the thicker pads were dragging on the rotors, causing the premature rear lockup and excess heat in the rear rotors.By the time I got to the pits I had bo pedal - brakes were gone.

Cooled the thing off - bled the system - fluid seemed unusually hot given how time had passed - 30-45 minutes.

Pumped the pedal and could not drive the car off - the rear brakes were locked on solid. Put car up on jack stands - Turned a rear bleeder and released obvious rear system pressure - could now spin rear wheels by hand - calipers had released.

With car still on jack stands - bled system again as pedal was soft- got a good pedal but again rear calipers were locked - brakes on. Fronts were fine.

Put it on the trailer and home we went.

At home bled all the old fluid out - replaced with new Motul 600 (same as was in the system)

Rears locked up as soon as I got to a "firm pedal"
Pulled the combination valve apart by removing the large nut on one end - pulled out a piston like item that has a rubber seal on it - inspected it all - could see no obvious issues so re-assembled it.

I replaced the Master cylinder, thinking the secondary piston, that applies the rear brakes, was in some was hanging up. Not so... the new one operates not different than the old one.

Now waiting for local dealer to deliver more Motul 600 before bleeding the system

So what am I missing - I'm afaid as soon as I bleed system and get a firm pedal the rears will be locked up again.

I haven't found anything obvious that tells me what is causing the problem in the first place.

Also have gone back to the original pads on the rear and dumped the thicker ones.

Any help is appreciated
Thanks

 

[HP]

Gavin, your rear calibers might have been heatdamaged from the first lockup. You might disassemble them and check the seals to see
if they're sealing properly. If not the piston won't retract fully as you let up on the brake pedal and they'll drag.

If ok, then maybe you didn't get a complete bleed and the air
in your lines is your problem. Sometimes a different bleeding
method can help, such a pressure bleeding from the calibers.

 

[GTS Dean]

I agree with Hugh. Sounds like you cooked something in the rear calipers - maybe piston seals. Rebuild and go back to some thinner pads.

 

[dblankenbaker]

If you are using an aftermarket master cylinder, you will likely need to use a shim between the the master cylinder and booster so that the piston retracts fully when the pedal is released.

 

[malcoll]

The problem is those Corvette rear calipers.

But seriously, I had a similar problem on my XJ-S Jaguar, the rear calipers got really hot once... it melted the seals and the rears locked up solid.

Pull and rebuild your rear calipers.

Lance

 

[Bruce Donaldson]

Hey Gavin:

Call Ted May @ Valaya (408) 448-8811. He's the Viper Tech to the Stars!

Works on a bunch of fast Corvettes too.

See yuo at Laguna Seca...

 

[K Adelberg]

Gavin,

Isn't this the Roe Racing SLR brake package? I am having a serious case of deja vu. When this happened to me we had to rebuild the rear calipers. The PBR (Corvette) Calipers are known to warp after exposed to high temperatures, not really a race quality caliper. I hate to say, but your problem is a propotioning one. The system is not balanced correctly. Attempts to educate the vendor were unsuccesful.

A cheap solution, is to rebuild the rear calipers, put back the thinner rear pads, check the brake booster piston to see that it can retract fully and cross your fingers. My opinion of this brake system, I believe is well known. Feel free to search the archives for my experiences. With this same brake system I experienced full rear lock up on several occaision. Good to hear you were not hit by another car. I had a close call and my car also caught on fire.

Good luck.

 

[SoCal Rebell]

Gavin, what are we gonna do with you? Get a set of Stoptechs like the rest of us, you had problems already at BW with Green Flag. Your fluid was getting REAL hot then also, when you bled your brakes with my evacuator you melted my plastic hose, something I never came close to doing with my '00 even right after coming off a track session. Also since yours is a Hennessey car you probably have a proportioning valve you can adjust, maybe it's turned the wrong way, good luck mate, no worries

 

[Jon 98GTS]

Hey Ron, I've got a riddle for you:

Q: What's worse than being "taken downtown to Chinatown"???

A: Ending up on "Skid Roe." Errr, I mean "Row."

(D@mn! I hate it when I make "typos...")

 

[SoCal Rebell]

What's wrong with Chinatown? I like going there, good food. And I especially like being taken there, that way I don't have to drive and can drink lots of Saki

 

[K Adelberg]

Dean, you should check your facts. I doubt all the parts are identical to what came off my car. Just curious, during your testing, what rotor and caliper temperatures were you seeing. Usually you are quite thorough in your testing, so when you say you did plenty, please share your data. McGuire's car may actually work since the system was closely examined, and probably modified to ensure the booster piston will retract. Sean did say my booster could not be re-used.

Could Gavin's car be just a bizarre coincidence?

 

[GTS Dean]

It was all SOTP testing. I had no pyrometer to check temps, but by the smell and the radiant heat coming off, they were working plenty hard. It is doubtful that your MC was used. I was playing Mr. Surveyor/Estimator that day and most of my driving was in the Ram.

 

[Mike H]

Gavin,......
Just went thru this problem with our car. The problem we found was nothing with the calipers. The Bias control and the rear brake lines were starting to weaken and collapse. The minute we disconnected the bias control the brakes worked fine. We found that our 4 piston Brembo front brakes and 2 piston PBR rear calipers worked quite well. I am amazed at all the Viper owners who say buy this, buy that and could care less to answer the question as why they caliper locked up. Brake setups that work fine for years need to be diagnosed as to why one day they lock up and the answer is not to go and buy a whole new setup.
We are now rerunning the braided lines with solid new Stainless lines, eliminating the chance that braided lines do collapse over time.
Hope this helps...

 

[Mike H]

Gavin...

What pistons are you running in the PBR's? Also eliminate the dust boots if you road race.

Ps: Some people I think should use the throttle more and the brakes less and maybe there brake systems would work...LOL

 

[K Adelberg]

Gavin, which rotors are you running.

Dean, which rotors were on McGuire's car when you did you "testing"?

 

[Gavin]

Ok guys - thanks for all the input - Yep I run Seans setup and as I said in my initial post it has been perfect in about three years of track events.

The problem was caused by me, by installing pads that were too thick.

I beleive this caused the little rubber seal in the proportioning valve (factory combination Valve is what it is really called) - to stick .

I pulled this apart, cleaned it, put it back together , had the brakes pressure bled and the system seems fine.

Will give it a further workout this weekend at Laguna Seca.

Ken - my brakes did not lock up on the track - I followed the great training given by great instructors. The moment I felt the pedal go a bit soft I backed out of it, and got the car into the pits. Put it on jack stands and there it stayed while I tried to fix the problem - I was not successful so it went on the trailer.

Ron - I will replace those hoses man - sorry about that - it does speak volumes about how hot I got those rears - or was that you sucking too ******* the hoses that flattened them???? Are you coming up to Laguna this weekend? Oh, That's right they have a sound limit - you won't be able to go round the pits yelling at us slow guys for getting in your way - they will meatball you!!!

Corvette calipers - I agree they are not ideal but at the time I bought my system, it was the only 4 wheel solution that I was able to find and from a guy I felt confident in his ability to support what he was selling. No regrets, had I checked with Sean before changing to another pad, he would have straigtened me out and saved me a big self caused headache.

He is developing an AP caliper for the rear, I have confirmed the vette calipers have not spread, completely dismantled them and reassembled and they seem fine - this weekend will tell more of the story.

Ron - I sure do like the look of that Stoptech setup - got any spare change mate!!!!

Anyway - thanks every one - I sure am glad I bought a Viper and get to be a part of this group. Happy holidays and looking forward to seeing a few of you this weekend.

Gavin

 

[GTS Dean]

Adelwhiner -

I drove Jon McGuire's RT-10 at the Area 69 facility in November. The place has a 10.1 mile, 32 turn road course and a 1000' long dead-level concrete pad (with a 2500' run-up) for brake testing - so I did plenty. The Roe brake system was perfectly balanced and performed without lockup and without slewing to one side or the other.

You want to know the most AMAZING thing about McGuire's brake system? It's the SAME EXACT "Death Brake" PARTS THAT CAME OFF YOUR CAR.