Brake troubles. Please help!

[TJ.Harry]

We've tried everything we can think of. We need your help.

I have a '74 with a 302 and C4. I did the Chevy front disc conversion (new parts) and added a new brake booster.

My current problem is a large lack of stopping power. The pedal feels ok (a little mushy) and applies pressure to the brakes but not enough to make me comfortable with it on the road. Even at as little as 20mph I can slam on the brakes and it creeps to a stop. It's currently impossible to lock up the brakes at all. It's bad enough right now that I don't trust it on the road at all.

I've replaced:
Master cylinder
Proportioning valve
Flex lines to calipers
Front pads
Interior rear drum parts

I bench bled the master cylinder and have bled the air out of the lines at the brakes several times. There are currently no visible leaks or compromises in the lines.

I don't know what else to do. Please help. I need to be able to stop. Thanks!

 

[Hozr]

Which M/C did you swap to with the disc conversion? Stock Prop valve? I prefer a disc/drum combo without the prop valve personally. Keep the circuits separate.

 

[krashnpa]

Make sure you check your booster vacuum source and check valve. Kind of sounds like lack of boost.

I also run an adjustable aftermarket prop to the rear circuit to adjust bias, but it sounds like boost or air.

 

[67crush]

Are the bleeders pointing up? If they are, try tapping the cylinders lightly with a small ball peen hammer while bleeding.

 

[Bonco4Broncos]

Some of the GM proportioning valves have a shuttle built in to send all pressure to front or rear in case pressure is lost on 1 end. I had the same issue when I bled the brakes after rebuild on a 78 camaro. I have been told to bleed a front and rear at the same time to avoid cycling this shuttle. I rigged up a water bottle and put enough brake fluid in it to cover the end of a piece of rubber tubing attached to the bleeder on a front caliper and a rear wheel cylinder. The fluid in the bottle acts as a check valve to prevent air from re-entering the system. Make sure to keep a check on fluid level in the M/C if you try this. The tubing I had on hand and used was old aquarium tubing. There are several videos on YouTube on how to do this.

 

[big blue]

This may help.
http://www.mpbrakes.com/troubleshooting-guide

 

[TJ.Harry]

It's the prop valve that WH sent with their front disc conversion. I have considered just eliminating the prop valve all together but I wasn't sure how that would effect performance if something else went wrong.

I have good vacuum for the booster, but you're right it does feel like it isn't really working very well.

The bleeders are pointing up. I'll try and knocking on the cylinders while bleeding. You think some air may be trapped in them?

I'm getting pressure to both the front and rear, just not enough for good braking power. I have considered trying to bleed the front and rear at the same time though.

I may try and isolate the front and back brakes like suggested on that link and see if I can figure out if it's one of those sapping the power. I would think that I would still be able to stop easily at 20mph though even with just either front or rear brakes alone.

 

[Rustytruck]

When you apply the brakes make sure there is still a gap between the caliper and the spot on the knuckle where you ground it. If these 2 parts touch the caliper will not clamp down on the brake pads.

 

[muskrat]

Interesting post, since I am expecting my GM front disk conversion parts from TBP's to come in this week. I can tell you that the guy at Tom's asked me specifically if I had enough vacuum to run the booster. Frankly, I don't know if I do or not. Have never checked my vacuum. Maybe that is issue?

I believe he said I needed 18" of vacuum for the system to work?

My engine is fairly stock except for the Edelbrock 0-5500 rpm cam, intake and crab.

Hope you get it worked out. I'll be interested in the responses from the helpful folks here.

 

[DJs74]

Have you tried adjusting the plunger rod running into the MC? It may be a little short and therefore not giving the MC the proper stroke needed to push the fluid to the calipers. My kit also had some adjustment at the new bracket / linkage mounted to the firewall.

DJs74

 

[Apogee]

Test the booster and report back, it's simple. Pump the pedal several times, firmly apply and hold the brakes with about 20-30 pounds of pedal force, start the engine and note the distance that the pedal drops when the engines starts. It should drop an easy 1/2" to 1" give or take depending on your vacuum level, booster size and health, MC bore size, etc.

 

[TJ.Harry]

I think the booster is working ok, but I'll test it. The front calipers are clamping down as I'm not able to turn the wheel with the brakes applied when it's off the ground. The plunger rod is an interesting theory I will have to check out. I never adjusted it (not even sure that it can be adjusted) and that would explain the symptoms it's having I think.

 

[DJs74]

I think the booster is working ok, but I'll test it. The front calipers are clamping down as I'm not able to turn the wheel with the brakes applied when it's off the ground. The plunger rod is an interesting theory I will have to check out. I never adjusted it (not even sure that it can be adjusted) and that would explain the symptoms it's having I think.

You should be able to adjust the rod leaving the booster / entering the MC. As I mentioned earlier, my kit also has adjustable linkage where it comes through the firewall (kinda inside your new firewall bracket). All the adjustments are geared toward adjusting / calibrating brake pedal travel to coincide with MC plunger travel distance.

DJs74