brake problems any suggestion?

I know someone posted a simular request a few weeks ago but i couldn't find it. I have a 55 chevy 2 door handyman wagon, i just put on speedways disc brake conversion and a 7" booster,i'm running a disc -drum proportioning valve and a adjustable line pressure valve to the rear brakes. My problem is that when i adjust the rod that goes from the brake pedal into the booster i either have a good pedal but the brakes are dragging big time or the wheels turn free and the pedal goes to the floor. any suggestions? thanks

 

My coupe had a similar problem. We fixed it by installing residual check valves in the lines. Make sure you use the two pounders for the disks, and ten pounders for the drums.

 

Not really sure whats going on here, but lets see what we can do.

First off those 7" boosters are pretty weak for your aplication, but that isn't the problem here. Have you checked the clearance between the booster and cylinder. There should be a minimal ammount of free play between the two, say .030 to .060". There are tools available at places like NAPA to check this or you can do it like we do and set it by feel. We do it with the master and booster off the car and the master loose from teh booster. Holding the two together the master should fit up against the booster without resistance. Holding the assembly together verticaly (master up) push the unit down on the power boosters pushrod. You should be able to feel the pushrod engage the poppet valve and then start pushing the master cylinder off the booster face. If you are pushing the booster down quite a bit before the master cylinder moves you need to lengthen the master cylinder pushrod.

Once you have that clearance set re-install the unit and connect the pedal. The pedal pushrod should not compress the booster pushrod at all. If your car still has the return bumpstop the pedal arm can contact it but there should be a little dayligt in there. If your adjustment here keeps pressure on the rod just like the above adjustment the pistons inside the mastercylinder will not return properly allowing the master cylinder to "refill" properly resulting in a constant application of the brakes and the dropping pedal as the brakes wear. This condition can cause the brakes to lock up as heat is generated and the fluid expands. When the fluid expands and the return ports are blocked the system will cause more pressure to be applied to the wheels causing them to lock up.

If the master cylinder and power booster is adjusted properly and you still have a low pedal condition, just how low is low in this case?? If the pedal is going to the floor then you have a problem somewhere else.

I have run into problems using the later model GM Metric calipers like the S10 and Camaro. They are made to be run with a GM Quick Take Up mastercylinder. If you have these calipers you can either rebuild them with an O-ring kit for a 1978 Monte Carlo/Malibu kit or just outright replace them for the earlier versions.

Other things to consider like rear brake shoe adjustment. If it is way out the pedal will have to travel more to take up the slack. We have seen many rear disc brake kits cause problems too. Most rear brake calipers will have an integral parking brake lever built into them. It is imperative to have that parking brake installed and operable and that you use it frequently. The parking brake rachets the piston out for adjustment. If it is new/rebuilt or never used the pads will be too far away from the rotor and cause excessive brake pedal travel.

 

i appreciate the input. the one thing i forgot to mention was that i replaced the master cylinder with another rebuilt one and this made a major difference, with the first master no matter how much adjusting i did either way made no difference. i'll try both suggestions, thanks

 

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I know someone posted a simular request a few weeks ago but i couldn't find it. I have a 55 chevy 2 door handyman wagon, i just put on speedways disc brake conversion and a 7" booster,i'm running a disc -drum proportioning valve and a adjustable line pressure valve to the rear brakes. My problem is that when i adjust the rod that goes from the brake pedal into the booster i either have a good pedal but the brakes are dragging big time or the wheels turn free and the pedal goes to the floor. any suggestions? thanks

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A proportioning valve and an adjustable line pressure valve?

Either one should be okay, but isn't the adjustable line pressure valve the same thing as a proportioning valve?