Hi everyone, it's been awhile since i've posted but I'm hoping some of you may be able to help me sort out this problem.

I have an 81 Bronco that I've swapped in a D60 front, Sterling rear, F350 master and booster, all new lines and brake hardware. I did the rear brake conversion with the RuffStuff brackets and chevy calipers.

My problem is the back brake calipers are dragging so bad that I can barely move the truck, once it is moving the rear pads and rotors get crazy hott.

I still have the stock 81 bronco proportioning valve...Does this hold rear line pressure?

I gutted the adaptor on the F350 master for the rear brake line so it doesn't hold pressure.

The calipers are good....I need help

 

First thing I would do is a little troubleshooting. Get it setup with the back tires off the ground, and do what you have to do to get them sticking and dragging. Once you do that, open the bleeders on the rear calipers. If that does release the calipers, then you know you have a hydraulic problem. If it doesn't, then you have a mechanical problem with the calipers or the brackets/parking brake mechanism, etc.

The next thing I would do(if it is a hydraulic problem) is set it up again and get it to drag again, and then crack the line at the master cylinder. If they release, then you know it's the master cylinder, if they don't, then suspect something inbetween the master and the caliper(like the proportioning valve).

If you make it all the way to the master cylinder as the culprit, the next thing I would do is get it sticking again, and then loosen the master from the booster and see if they free up. If they do, the rod between the master and the booster may be too long.

 

Some really good advice, I have done some of this already...

 

Keep cracking the lines at different logical places to narrow it down.

 

Does anyone have a breakdown of how the proportioning valve works on an 81 bronco...I think its my problem

 

I was looking at doing a rear disc brake conversion on my 86 F150, and in my research of this task, I read that the proportioning valve will need to be changed, as the discs and drums require different pressure. One site also suggested that the master cylinder may need to be swapped. For now, I'm going to stick with upgraded drums out back. Good luck on getting this sorted out.

 

Are you still using the original master cylinder? IIRC, a master cylinder designed for use with drum brakes has a "valve" in the master cylinder that holds about 5 psi in the brake line to the drums when you let off of the pedal. When you switch to disc brakes, you have to remove that valve so the pressure drops to zero.

It may not sound like a big problem, but with that pressure in the line, the disc brakes drag a little all the time and it causes the calipers to overheat and fail eventually.

 

I first did the disk conversion and kept the stock bronco master and booster. The back calipers stuck with this setup. I then removed the stock stuff and installed a master and booster from a mid 90s F350, same problem rear calipers stick. I then noticed the F350 master had a large nut threaded into the rear line which I believe holds pressure...I took that off and gutted it, reinstalled it and same problem with back calipers sticking. I'm almost convinced there is something in the factory proportioning valve that is causing my problem.

Does anyone know how the factory proportioning valves function...can you take them apart and open them up or anything?

 

Bypass the proportioning valve.

 

See if this helps you:

https://www.ford-trucks.com/forums/1...lve-issue.html

 

I am starting to think that is my best option....gonna hit up summit to look for an adjustable PV.

 

Thank you!! It does help, makes me think that something in mine is stuck or not functioning properly.

 

I know it's probably a stupid question but, are you sure it's the calipers and not the emergency brake? Also, bypassing the PV isn't really a good idea. That's what insures that the rear brakes don't lock up prior to the fronts. And yes you really do need a master cylinder and PV from a disk/disk setup not a disk/drum unit.

 

Exactly!

It may not be a good idea bypass the proportioning valve for good, but taking it out of the circuit to do some troubleshooting is a great way to see if it's the problem is being caused by something in the MC/PV area or someplace in the rear wheels.

And, yes, the setups are different for 4-wheel disk versus disk + drum configs.

 

If I take out the stock pv I would add in an aftermarket adjustable pv so the rear brakes don't lock up. What master cylinder came in a Ford truck that controls disk/disk?