brakes starting to drag.. why?

Aftrer about 500 miles of driving I noticed that my brakes are starting to drag, progressively getting worse, I cracked the bleeders on the two rear calirpers as they were the easiest to reach at the time, just trying to do some basic troubleshooting and it rolled freely after that. The right caliper released just a small amount of fluid and none out of the left. After driving about 75 more miles I can feel it starting to drag again. Fronts are disc and rear are disc, universal 7'power brake ***embly, 2lb residual valve front and rear, adjustable porportioning valve, everythingf is new lines etc. Pedal has about quarter inch of play before ingaging m/c. Looking for ideas to fix this baby. Thanks Mark

 

Aren't 2 lb valves for drum brakes? How old are the brake hoses?

 

Discs called for 2lb valves unless I read wrong which is possible, all the lines everything is new, about 500 miles or so. Thanks Mark

 

Good point here. Had an application where the hoses were supported by a bracket, the bracket was growing rust on the inside and pinching off the hose causing it to act like a check valve.

 

The booster may still be holding the masters piston from returning even though there is play in the pedal. 2# residual valves are correct for discs.

 

Landseaandair how would it be doing that? Not saying your wrong at all ,Just don;t understand how,,,, bad m/c or booster, pedal not adjusted correctly. Even though I have some play I do not have a return spring on the pedal. Could the weight of the pedal even though very little be a problem? thanks Mark

 

I personally saw this happen on a 60s Impala where the new replacement boosters internal pushrod was too long. Checked it by pulling the master foreward a bit and compared things with a gauge whipped up from some posterboard. Had some adjustment in it but still required some minor t******* to get it right. I suppose the weight of the pedal lacking a return spring could do it too, especially with power brakes. Add one and see, also could be debris in the masters well, blocking a return hole.

 

I've also seen a new booster with sticky valving that dragged the brakes when cold.

 

Try putting a .040 shim between the MC & booster to see if the booster push rod is too long. You can use thin washers for the shim.

 

Thanks for the replies and ideas, I'll try these things this weekend if all goes well, thanks again. Mark

 

Does the master cylinder have a built in residual valve ?

 

With full vacuum (about 20 "Hg) there should be a small clearance between the booster push rod and master cylinder primary piston, usually '020"-'040".

I'm not familiar with aftermarket boosters, but on factory OE units the pedal push rod has no visable clearance adjustment, as the push rod enters into and becomes part of the control valve. The rod simply connects to the pedal that is returned with an external spring, making the booster and pedal ONE. Not sure how you can have any pedal to booster clearance or measure any pedal to m/c. Maybe a picture would help.

I would 1st make sure the booster (pedal) push rod is fully releasing and staying in that position with your foot off the pedal. Make sure you have a separate pedal return spring, and not rely on the internal booster spring.

 

Is it better cold and worse hot or just constantly dragging? How long did it take to start dragging again after you released the pressure? It kind of sounds like you have air in the line. If you opened the bleeder valve and nothing came out, there might have been air in the line. If you didn't have something in place to ensure that no air came back in when you opened it like a foot on the break), it's likely that there's air in it now anyway.