1986 GXL with rebuilt 87 TII engine.
~1000 miles on engine
361hp on 14PSI bogging out rich.

Car has a new master cylinder, new front calipers, rebuilt rear calipers, and I replaced all of the hardlines on the car too.

Rotors are new Advance Auto OEM blanks.

Pads are Hawk HPS all around.

I have a motive power bleeder and bled the bejessus out of the brakes. They felt great!

Took it to Deal's gap and beat on the engine pretty hard, but didn't really stress the brakes that much. After one run, my buddy in a WRX had his brakes at 950 degrees, mine were at like 230. I don't think they even smoked all weekend. The pedal feel, however, changed dramatically.

Instead of a nice progressive feel when I first started the weekend, after a few runs (which I didn't use the brakes much on), the pedal got a bunch of travel in it and then got REALLY stiff and I had to put a ton of leg into it to get the car to stop. It also felt like the brakes lost all bite and were hot, but I doubt they were that warm.

I'm wondering if boosting is damaging the brakes more than braking is, because I didn't beat on the brakes that bad.

I have a vacuum manifold run off a 3/8" line which feeds 5x 1/8"NPT lines and the brake vacuum assist hardline off a 3/8" line. The brake vacuum assist hardline runs to the stock hose and one-way valve.

What say you? How do I troubleshoot from here? Rotors look good still, crosshatch and all, I don't think I glazed anything up. I don't think I quite bedded in the pads correct though, either. I don't know.

 

Sounds to me like the Vac assist diaphragm has got a leak. Check to see if there is brake fluid in the vacuum line from the manifold, if there is that is a sure sign that the diaphragm is busted.

 

Okay, I'll do that tonight. Brake fluid in the manifold is probably not great for my map sensor or wastegate or BOV either *facepalm*.

I should also mention I am feeding the manifold off the large vacuum source coming off the back of the manifold where the BAC originally mounted.

 

Are you sure your one way check valve is turned the correct way (air flow away from the booster)?

I swapped my na to TII and found out under boost my brakes did like you described above. While in boost I could not push the brake peddle down. My car was lacking a check valve so I ordered a 3/8" check valve off ebay and wallah, problem solved. It sounds to me like you have your check valve installed backwards, or it's not functioning correctly.

 

+1 on the check valve. I forgot to put mine in after a rebuild and right off boost you have NO brakes. Kinda scary the first time it happens. You said yours gets worse over time though.

 

I left the check valve the way it was on the car when I did the swap, plus they were working excellent before this last weekend.

Allow me to add some more details.

Just took the car for a run. It was running fine, albeit awfully rich. I noticed I'm hardly pulling any vacuum...I usually pull around -20inhg (-9psi) at idle, it's down to about -10inhg. (-4.5psi) This has shifted my fuel map so that it's bogging out rich at cruise.

I thought, oh, this is perfect...the brake booster is leaking, causing my brakes to suck and the car to run with an awful vacuum leak.

I got home, preparing to pop the vacuum line off the booster and see brake fluid inside. I pulled the line off, and not ONLY was it bone dry, but when I popped the hose off, there was a WHOOOSH noise as all the vacuum contained in the booster escaped.

Booster appears to be working great.

Any other ideas?

 

So, the line that feeds my vacuum manifold is apparently too large for the nipple on the intake manifold.

I messed with it and my idle went from -4.5psi to -7.5psi. I was losing 40% of my vacuum through that hole.

Assuming I was also losing 40% of my boost through that hole, my MAP, BOV, wastegate, and boost gauge were also reading 40% low.

Which means that wastegate spring set to open at 13.9psi was actually opening at 19.5psi...and the haltech was only injecting 13.9 psi worth of fuel and not fuel cutting at 17 psi.

****, that's scary. I'm honestly a little surprised I didn't blow it up. I'm going to pull the manifold off and pull vacuum from the OEM bleeder source, it looks a lot bigger.