Brake issues....stumped!

[bill]

Ok...so I had carb issues which caused me to have to tow the car home with a tow rope...just a mile or so, but without the motor running, the brakes were all but gone! I nearly ran into the guy pulling me with the truck. Anyway, after fixing the carb issue (stuck power valve), I noticed my brakes were still hard, like I had no power booster. So, I checked the valve, checked for vacuum and decided the problem was the power brake booster. And since I was at it, I bought a new master cylinder too. Replaced both...no help. Then I read that sticking wheel cylinders can cause a hard pedal, so I rebuilt the rear brakes, replaced all the hardware and the cylinders, shoes etc....still no help! So I bleed the whole system once more, and still having strange behavior. The brakes work fine sometimes, but if I pump them several times as I am driving, the pedal gets hard and the brakes dont seem to grab. I replace the calipers about a yr ago and the car hasnt been driven or outdoors much at all since then. I noticed the old master cylinder had a lot of dirt in the reservoir. Is it possible the calipers are sticking? When the brakes are working, the car stops well, no lockup and no pulling to one side... I feel like it could also be a vacuum issue to the booster. I tested the port and Im getting good vacuum to the booster at idle. Cant tell what it is at speed, but the issue seems more pronounced when Im coasting and moving slowly (under 30).
Any ideas? Im usually pretty good when it comes to brakes, but this problem has me really frustrated. Spent the whole weekend and today working on this...and I still feel like Im not getting anywhere. Thanks in advance for the ideas, thoughts, suggestions......

Bill

 

[455rocket]

I would change the check valve just because, can't be more than $10. If that don't do it I would take the booster back & get another one. It's a damn shame but new doesn't mean good anymore.

 

[bill]

Its a mutha Flocker to get on and off....Im trying to eliminate all the other possibles first before I start pulling parts. Funny thing is, when i pulled the check valve from the old booster, it was holding vacuum in the booster, which got me worried. Im pretty sure my old booster was ok. Chilton says if you have a soft pedal, followed by a hard pedal, the possible causes are sticking calipers, wheel cylinders, or power brake unit binding, master cylinder compensator port blocked. Would a bad proportioning valve cause this? Brakes have been good up till I rebuilt the carb a few weeks ago and had issues with the power valve sticking. From that point on, the brakes havent been the same.
thanks for the suggestions....

 

[LS1GN]

Did you have backfiring through the carb? Did you damage the line to the booster?

I think there is a 99% chance it is under the hood, not the brakes/fluid/calipers.

 

[bill]

when the power valve in the carb stuck, it flooded it badly. Had to take the carb apart and fix it. runs fine now. got good vacuum to the booster, 20+inches, but its fluttering on the needle a bit. Think I have a few worn valve guides. brakes work fine if I am running down the road and need to stop, but if I am coasting, say at 30mph or less, and hit the brakes, the pedal is hard and I have less stopping power.
Heres a good link I found....
http://www.classicperform.com/TechBook/ ... m#testprop
Im gonna check the proportioning valve and see if its sticking, then Im gonna check the calipers to see if they are sticking. Then Im gonna tune the engine again, reset the timing, fuel/air mixture, and idle.....Thanks!

 

[bill]

update: I realized the other day that I had the distributor advance hooked up to manifold vacuum rather than ported vacuum. This means that I was getting advanced timing at idle/low speeds, which not only made the car run funny, but increased the vacuum on the engine to above 20inches, which may have been sucking the hose to the booster flat. I changed the vacuum advance hose to the ported side of the carb, and the idle vacuum dropped about 6inches. But, because the vacuum advance is now only coming in at cruising speed, the vacuum on the engine stays more constant, which gives the booster more vacuum when I need it. Took it for a drive and the car runs much better and the brakes appear to be more consistant under all conditions. Im still gonna check the prop valve, just to be sure its good.

 

[bill]

I have a dumb question, but I just gotta ask it..... What effect does running a motor without exhaust hooked up have on engine vacuum? I can assume that without an exhaust system, theres almost no back pressure to create a vacuum...any thoughts?

 

[Bowiemontecarlo86]

My friend use to run his Monte down the road with just headers on. He never had any problems with stopping.

 

[Bonnewagon]

Bill, I think you are onto something with the vacuum hose. What kind did you use? You need a thick double-walled hose that won't collapse under high vacuum, like when decelerating. A regular fuel line hose is meant for positive pressure, not negative. I get it at good Marine stores and it's not cheap, but it is fat and strong.

 

[bill]

Bill, I think you are onto something with the vacuum hose. What kind did you use? You need a thick double-walled hose that won't collapse under high vacuum, like when decelerating. A regular fuel line hose is meant for positive pressure, not negative. I get it at good Marine stores and it's not cheap, but it is fat and strong.

thanks bonne.... Your right on...Im using fuel line.... didnt find any vacuum hose that size at AZ...so i went with what I had available....I will look into the other hoses!