Now that resto is mostly done, been driving the car more often, and had to make a few hard stops. Im starting to realize there is a problem. During normal driving with light braking everything feels good, but during hard braking its a little scary. The pedal goes about half way down and then just stops. No matter how hard I press the car wont stop any sooner, Ive practically stood on the pedal and the brakes wont lock. The entire brake system is new, with front discs and adjustable proportion valve. Could this be a vacuum problem, the cam has a 292 duration and I dont have a vacuum canister? Any help appreciated!
Thanks, Rob

 

I ran into the same problem with my friends Vista. He also put everything new and had poor braking/hard pedal. I hope you still have your original parts for comparison. We found the after-market calipers were not machined correctly at the bottom of the banjo bolt hole. Check how the caliper, line and bolt fit.The bolt would almost bottom out when tightened. The tip off for us was the first thread on the original bolts were slightly galled when removed. I have seen different bolt lengths, banjo hose end thickness, washer thickness and incorrect machining of caliper. Stack a few of those together and you have a problem. We had good pressure at the caliper, but low volume. Bought a set of reman's with original GM markings and car stops as it should.

Jim

 

Had basically the same problem with the nephews car - 496 [?] stroker!
He's really borderline, and should get a vacuum tank!
He locked the timing [no centrifical advance] @ 30 - 32, and adjusted the carb, with a vacuum guage, to achieve the highest vacuum, I believe 14 psi.
Just hope you don't have to stop quick right after jumping on it!
I'd get a tank and be safe!

 

If you resto'd the whole system any chance the link to the booster may not be adjusted properly?

 

Not quite sure what the link is. Is that the link to the pedal?

 

Thanks Jim, will take those bolts out and have a look. Car originally had drums all around. I bought a conversion kit from Jeggs and had it installed by a pro.
Also how do check pressure and volume?
Thanks, Rob

 

We used a brake pressure kit made by Snap On. Pressure was easy to verify, low volume was based on what it took to repair the brakes. If the engine/cam combo was in the car prior to the brake upgrade and the brakes were working properly I would not suspect low vacuum.

 

Not necessarily, the car had drum brakes and the drum brake booster doesn't need as much vacuum. You can add discs with the drum booster, master and valve and they'll work fine with low vac.

It sounds like low vacuum to me, no vacuum reserve can will fix that and those cans aren't worth two cents in real world driving conditions anyway. The only things you can do to safely remedy things to where the car will stop dependably is to change the cam, switch to manual brakes, or add an auxiliary brake vacuum pump. The best pumps are the type they used in GM 80's four banger models IMO.
I have one in my GTO and you'd never even know it was there.

The first thing you need to do is check the vacuum at idle to see what you got then go from there.


http://victorylibrary.com/mopar/pump-tech-c.htm

 

Thanks for all the input. Going to take a vucuum reading and check those banjo bolts hopefully by wednesday, supposed to be 60 here. Will post my findings.
Thanks, Rob

 

In my 73 it has an adjustable rod/clevis (link) between the booster and the brake pedal. Not sure if that could play into this. Something to check if the other posts don't pan out. Good luck with the brakes.

 

Took a vacuum reading. We have 12-13 at idle, about 15 at 1k and then a max of 20 at 2k. This is a new engine with new brake system. Also took a look at the link, seems sucure at the pedal. Also my initial timing is about 15, any more than that it starts to ping.
Thanks, Rob

 

Is the check valve on the booster holding vacuum?

 

Is that the plastic piece on the end of the vacuum hose that plugs into the booster? If so, there is a rush of air when I pull it off with engine off.
Thanks

 

Yes, the check valve plugs into the booster grommet. The rush of air sounds normal, seems to be working. Maybe if you're still playing with this problem later you might try replacing it, just in case. There could be a slow leak thats hard to detect.