944hal

Having a problem with the brakes on my '87 951 track car. Here's what I recently installed. New front rotors, rebuilt turbo S calipers, new Hawk DTC-60 pads front and rear, 3" brake cooling hoses and intakes (installed in the grill under the driving lights) and backing plates. Brakes were bled multiple times to get the air out. The normal brake pedal is firm, not spongy. Here's the problem. The both times this happened was about 15 mins into a track session. Very hot out, near 100 degrees. Brakes were fine with medium to hard braking using smooth initiation, no sign of fade. Twice, I had to brake hard quickly, and both time the pedal seems to depress very slightly then kick back rock hard. I pushed as hard as possible to get minimal braking. Both times the brakes came back fairly quickly using minimal initial pedal pressure. Fluid is all brand new DOT 4 and there is no sign of fluid coming out the top of the resevoir as if it had been overheated and expanded. Clutch is fine. I'd appreciate any ideas. I'm hoping somebody out there has had a similar problem. I have an idea, but I'm fishing for confirmation.

Thanks.

shiners780

Swap out the brake booster check valve with a known good one and see if it happens again. It's a quick/simple/cheap test.

Chris White

Yep, sounds like you are loosing vacuum assist.
Braking sucks when they don't suck!

944hal

Thanks guys. The brake booster was also my first thought but I was hoping for something simpler. The good new is I have a couple of months before my next event. But, tell me more about the check valve. Where is it? Is it the culprit or will swaping it out only tell me something else failed?

fast951

Are the HAWK DTC-60 same as the "Blue"?
I run the Blue pads and I noticed the same thing on my car. Usually on the the cool down lap, between RA turn 6 and turn 7. It feels as the pedal goes down lower than usual, but recovers instantly (no loss of braking, just lower pedal initially). I wrote it off as pads related due to temperature, as I didn't have the same behavior running different pads.. Or maybe the pads are so retracted away from the rotor, that it takes a bit longer to move them closer to the mating surface.

It'll be interesting to see what you find out.

prossi

as stated try just the valve not the complete booster. also i didnt see mentioned if you have rubber brake hoses or steel braided. if you have steel braide3d did you change the clutch line also?

Oddjob

Check valve is plugged into the brake booster on the inboard side of the master cylinder, near the heat shield. Has a 5/8" hose connected to it (hose runs along the firewall, then routes under the intake and comes up between the #2 & 3 intake runners and connects to the manifold).

The DTC60s are a more aggressive pad than the Blues (9012s). Progression of the Hawk track pads is 9012s, HT10s, DTC60s, DTC70s (most aggressive).

Lemming

No.

944hal

Sorry for the delay. Spent yesterday helping a fellow racer remove the motor from his 951. Cracked cylinder wall. May be looking for a decent 951 short block (hint, hint). Back to the brake issue. The DTC-60 are different than the Blues. After some track time with them I wouldn't call them more aggresive, at least on initial bite, but I like the feel. They are suppose to newer technology?? Not suppose to need slotted rotors?? I'll get back with a review after I use up a set. They seem to be a lot dirtier. Hoping they are more rotor friendly than the Blues. Sombody else may have more experience with them. They were recommended by an extremely competent PCA club Racer from Knoxville, TN. My brake problem wasn't on the cool down lap. Hard late brake at about 135 and 150 feet from RA 10A. Slight pedal depression and then a hard pedal. Found out I could get through 10A faster than I thought!! I have steel braided brake lines. The clutch line is metal. At least the end that connects to the slave is. I found the check valve. I have a S2 street car. I going to pull the vacuum booster line, plug it and see if it feels the same. Two other things. I found a disconnected vacuum line. It goes to the device next to the engine fault code plug. I think it has to do with the fuel tank vapors? So I did have a vacuum leak. I also do not have a vacuum resevoir on the car. Usually mounted to the right of the battery facing the firewall.

xupkid2

One other thing to check is the pads to make sure the backing plate did not separate from the pad. I have had problems with Hawk's before since they are held on with adhesive vs pinned like Pagids. I spent hundreds of dollars replacing master cylinders and bleeding brakes only to find out the pad had separated. Now I run Pagid Blacks and have no issues at all.

944hal

The pad are brand new, first time out. Brake problem was 2nd and 4th session, but I'll check them out. Thanks.