Dragging brakes

[Shamu70]

Hey guys. Last fall I put on a new master cylinder on the 70 Newport. It’d been converted to front
disc brakes before I got the car, so I knew about the different sized resovoirs. Things have been fine
until my last 2 drives; it’s only when I back into the garage that there’s a dragging problem. And it’s
definitely (at least) the driver’s front.
Haven’t touched the proportioning valve (don’t know much about brakes, assuming there is one) and
need some pointers. Thank you.

 

[57fury440]

Is it possible the rod on the back of the master cylinder is too long and keeping pressure on the calipers?

 

[Shamu70]

It is. I noticed when I put it on that some kind of nut is on the end of the rod; I assume it can be turned.

 

[3C's & a D?]

How did the fluid look when you changed the master? Sounds like your caliper isn't fully retracting.

 

[Shamu70]

Clean…. because I was always filling it.
Leaked at the booster.

 

[Davea Lux]

Clean…. because I was always filling it.
Leaked at the booster.

First off, if a new master cylinder has been installed, that usually means that the travel rod for the booster will need to be re-adjusted. There is an acorn nut on the travel rod that is turned in or out to set the length. Remove the master cylinder to adjust the rod. Before pulling the master cylinder, check how much free play you have on the brake pedal. You should have about 5/8" of travel on the pedal. If you have enough free play, the problem may be with the caliper. If so rebuild both front calipers. If you had a large fluid leak from the rear of the master cylinder, this will usually cause fluid to enter the booster. This in turn degrades the internals of the booster and will cause it to fail in short order.

Dave

 

[Just Carbs]

If it's just the driver's side, and the driver's side runs hotter than the passenger side when checked with the temp gun after a trip around the block, I would suspect a clogged rubber hose, corroded caliper piston, or binding caliper slide.
If both sides run hot, I would suspect the presents of a 2-psi pressure maintainer valve in the master cylinder disc side that's intended to keep the pads close to the disc when the pedal is released. If it has one, it may be holding too much pressure. I remove them.
Or, the disc are plumbed to the drum side that has a much higher-pressure maintainer valve. But that ain't it because the problem is new.
Or the master piston(s) is dragging. The master piston should need very little air pressure to blow the piston out for disassembly. Then after it's been put back in and exercised at full travel a few times it should slide nice and easy.
I doubt they get a full travel exercise when they are assembled new.