Well I’m back with another “ simple” problem I have yet to resolve. My brake system includes : 1” bore corvette style MC. Front calipers with bore of 3.1 “. Rear wheel cylinders with bore of 1.06”. Pedal ratio of 5.727 ( manual system). problem: brakes feel ok unless you wanted to REALLY stop, then they disappoint. Pedal doesn’t feel soft , but if you push the pedal hard enough it will bottom against the stop. Wheels won’t lock or skid. Bled the system several times, no air visible on the most recent attempt. Rear brakes are adjusted tight. Entire system is new expect for OEM proportioning valve. possible air in MC ? It was bench bled before install… but can’t think of much else. thanks in advance for shared wisdom
All brand new pads and shoes? They will need a little time and heat to bed to the discs and drums. That is after the shoes have gotten the proper arch. Pop a rear drum off to see how much of the rear shoe is actually touching the drum. If very poor contact, it’s one sure reason for poor stopping. Hard to find shops with arcing grinders now though.
disassemble and C- clamp the wheel cylinders and calipers so they can't expand and see if it feels the same. If the pedal is firm, then you know it is the pads, arch, adjustment like suggested Is the master mounted on a solid bracket? I had a car in the shop that the bracket was strong enough to exert only so much brake pressure then would flex. new rubber hoses and are they quality made? I worked on a triumph and the only cure was to change to braided race hoses to get a good pedal, the new reproduction hoses would expand.
Thanks for all the ideas. The rear end is a full floater 14 bolt so it’s an effort to get the drums off. When I first installed new drum on new shoes, I walked them on with the shoes adjusted as tight as I could but still get the drums on. To adjust, I have jacked up the rear and turned the star adjuster and visually watched the clearance between the shoe and drum, getting it very close, but stopping if the wheel would turn by hand . The entire system has about 200 miles on it now. the brake lines are are steel or braided steel. The MC is very solidly mounted, been through that before. OEM proportioning valve is for the same system, but it was power brakes from the factory ( 1979 gm 3/4 ton).
Every time I have encountered your situation, I have found an air bubble hiding somewhere. It usually wasn't easy to get rid of. If you pump the pedal is it hard right away or are you still able to depress it?
I just tried it. I’ll admit it feels a little better after 2 pumps than just depressing the pedal. maybe a bubble trapped in the MC or the proportioning valve. I’ll try tapping those with a rubber mallet while bleeding
I'd check the disks for runout. It's possible that the pads are suffering 'knockback' and just needing additional fluid to move far enough for max stopping pressure. Especially since you state that it hits the stop. May not be the problem but it's quick, easy to do and free.
On adjusting the rear brakes, Tighten the adjuster until you get to the point that you cannot turn the tire by hand and back off the adjuster 11 clicks. I've adjusted hundreds of sets of brakes that way as that was what I was taught in 1962 and it has always worked for me. You have to have some drag on the shoes for the brakes to work properly. As for pumping the brakes to bleed them it is imperative that your helper pumps the pedal SLOWLY and holds the pedal down until you tell them to release it and is all but impossible for one person to pump bleed the brakes by himself no matter what the guys down at the hangout say they do.
No other valves.. New rotors, no pulsation, but you’re right, that’s easy to check. if anything, my adjustment might be too tight. But I’ll try your adjustment method next time I’m under there. I too was taught to pump slow. Everyone of my “helpers” thinks faster is better !
Good video. Never had a problem with what is basically an OEM system before, but if a 10 lb residual valve would fix my problem, who cares ? My MC is on the firewall by the way.
Follow up: I installed a 10 lb residual valve on the rear at the MC. Bled the brake system ( again) including the proportioning valve. ( no sign of any air) . I rechecked my clearance between brake pedal push rod and MC piston / cup. I closed it up a bit to a guesstimated .020. The brakes have improved. I am hopeful that with continued derived the rears will “ self arch” and pedal feel will only get better. thanks for all the ideas.
Does the proportioning valve have a check valve in it? Many of the "factory" style proportioning valves have a check valve that will slide to the front or rear if you have a leak and shut off the "leaking" side of the system, even if you have a dual master cylinder. The problem that can occur is that when you are bleeding the brakes, the valve will shift and shut off that section of the system because it thinks that it is leaking. To overcome this, you first (before bleeding) need to insert a tool that locks the proportioning valve in a "center" position so that it doesn't move to close off either the front or rear of the system. One way to check this is to see if both the calipers, and/or drum brake shoes actually move when the brakes are applied. If one end of the car doesn't move, the proportioning valve has probably shifted and needs to be re-centered and locked into position during bleeding. Don't forget to remove the tool and unlock it when done.
Give this a try. Break one of the lines loose right at the master, and have someone depress the pedal as you hold a rag under the line. Then keep the pedal depressed until the line is tight. Repeat for the second line. Often if there are any bubbles in the system after bleeding they are right at the outlet fittings on the master, and this works to improve the braking. Done it several times when nothing else worked.
I did include the master on my bleeding process. Unknown if any air came out as I only cracked the lines. Same with proportioning valve. Drove it yesterday, it has never felt better. I’m optimistic it will get better as rear shoes bed in .
If that piston has moved in the PPV, you can put a test light on the brake switch on top of that valve. If the switch is grounded the piston has moved. That is what turns your "brake" warning light on in your dash to alert you of a malfunction or leak. The test tool is plastic, replaces the switch during bleeding. Cost about $5
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