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Front&Rear disc w/out booster/weak brakes

1K views 14 replies 9 participants last post by  Frisco 
#1 ·
Fellas, I knew if anyone could help it would be you guys. We are racing a g.m. car on circle track(bought race ready/lol) After many hours getting this ready car rereadied we have come up on a stumper. I believe this is a problem you are used to. The car has
an aftermkt rear disc system welded on the flanges using g.m. 88 truck calipers. the booster is removed also the p. valve. the m/cyl was rebolted to the firewall The pedal has no freeplay.
We can't get a good pedal no matter what or how much bleeding
we do. on the 1st hit pedal goes to the floor by the 2nd a good pedal is up but braking is weak no matter how hard its pushed.
I noted the rear calipers are 1/8th inch nonparallel to the rotors.
They center and barely engage on the 1st hit and fully clamp the 2nd. Would a m/cyl for a disc/disc system help . I hope you drivers and straight liners can help a poor old dirt tracker.
Thanks bob
 
#2 ·
All I know is that when a brake booster goes bad or the engine
dies, the brakes don't work worth a flip.
So IMHO taking the vacuume booster off somthing that was set up for it, gives the same effect.

I think you will have to make the brake pedal arm longer or use
a brake booster.
I'm not sure if vac boosted master cyls are different from non boosted master cyls.
 
#3 ·
You are going to have to brush up on brake theory. If you are using disc/disc, it will require a master cylinder built specifically for the system, especially without the aid of a vacuum booster (larger piston bore). You are also going to have to verify correct pushrod adjustment.

I can see no way that you can run a system without a proportioning valve/metering valves or proper combination. The designer had the correct concept but went about it incorrectly.

Check -here- for a quick course in brake theory.
 
#8 ·
If you are using the same pedal as was original with that m/c, it will take a considerable amount of force on that pedal to achieve the same braking without the booster. It would be similar to having the engine die while driving. The porportioning valve being left out is also a concern. Even if you had good brakes(holding force), they would likely not be suitable for driving. The force needs to be biased (proper front to rear relationship)for proper control.

Like was mentioned, you might want to read up on brake theory, and go from there.
 
#9 · (Edited)
The crooked rear brakes will make you have to pump the pedal up no matter what else you fix. The rotor is acting like a lever pushing the piston back in. Your disc drum master is already holding about 7psi residual pressure, which should be too much, but even that cant overcome the mechanical leverage of the rotor trying to force the caliper into a straight position.

And bearing repeating, read up on theory.
Manual brakes require more leverage. This means you have to adjust and move the pushrod upwards on the pedal arm (this will require drilling if there was no option for manual brakes on that vehicle). You also need a master cyl which has the correct bore size for your chosen system, and as said, a quick takeup, if it is called for, or a slight residual valve to go with a regular disc/disc master.
You need a prop valve or your bias may be off.


Edit: I forgot... A parking brake activated piston will require adjusting to keep the pedal from having to be pumped. This is done either through the parking brake lever on the caliper, inside the car if it is connected, or sometimes there is an adjuster under a plug on the back of the caliper. If you cant seem to be able to get it to adjust all the way because it is crooked, unbolt the caliper, and adjust the piston out to where the pads just barely brush the rotor parallel, and then get the caliper and the wheel bolted on. If the caliper wont line back up to the bolt holes, or the wheel is locked up or dragging after everything is back together, it is too crooked. Really, any crooked is too crooked, but you get my meaning. If you do get it adjusted all the way out without unbolting it, be sure to check the wheel for drag after bolting it back on.
I really dont know if those are p.brake activated or not on those. Most GMs are. I am also assuming hat style pop on rotors, which require the lugnuts to be on to be sure everything is straight. I should have thought of all this first. Sorry.
 
#10 ·
They center and barely engage on the 1st hit and fully clamp the 2nd. Would a m/cyl for a disc/disc system help .
I would say here you have the wrong master cylinder. You need a disc/disc with the reservoir capacity to move all calipers at once.

And yesgo made a very good point. You may have to set the air gap on the calipers (initial distance from pad to rotor).

You will most likely have to refer to the correct shop manual for the rear discs for the correct procedure.

 
#13 ·
much thanks

You guys were pretty much right on in your suggestions, thanks.
As a side note we have adjustable proportioning valves. installed
in the ****pit. Of course I have no idea what type m/c is installed now but have ordered a 79 vette m/c to try and alleviate these
problems. There is currently no airgap on the rear rotors except for the alignment issue and I also bought a residual 2 lb for that.
Actually the minor lag time for rear take up may work as an advantage on a tacky track where you want the rear to come around somewhat. Will post an update when completed.
 
#15 ·
Re: fixed

bobaluuu said:
the m/c and residual valve fixed it all, thanks a million for tryin to help an old dirt racer.m bob



Light it up, Baby!
Glad to hear that you have solved your problem. :D

I read your post late, but here is a link to an excellent article concerning brakes and disc brakes with and without a power booster.

http://www.hotrodheaven.com/tech/brakes/brakes1_index.htm

There are eight pages plus links to related info.
 
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