Disc brake question
 

Had Front disc brakes put on my 1950 ford F1. ( local shop did it). Here is my problem.... I'm having brake drag. When the truck sits over night, 1st thing in the morning I can jack it up & front wheels spin nicely. After a few miles of driving, brakes start to drag. bring it home, jack truck up, wheels will not spin at all. anyone have any Ideas?.. I still have drums on the back. Thanks .. Marko.

Re: Disc brake question
 

Did you install a proportioning valve?

Look here:
https://www.google.com/search?q=prop...hrome&ie=UTF-8

Re: Disc brake question
 

Disc brakes perform better when hot. In part because of expansion.

Likely fix would be to adjust the MC push rod. Not being familiar with your equipment I can't detail the procedure. However I can tell you there should be a provision to adjust the length of the rod. In your case, shortening the rod would give you the desired effect. It shouldn't be much. You will be pleased.

Re: Disc brake question
 

Was a disc/drum brake master cyl installed? I had this problem years ago. Turns out there is a small flapper valve in the ports of the master cyl for drum brakes. Its there to hold residual line pressure for the drum system. Once I pulled the line seat and took the valve out all was good, just didn't have the fluid volume of a real disc brake master cyl.

Re: Disc brake question
 

This is good advice. You do not have enough "free play"in the pedal. Encountered this years ago when I had put hydraulic brakes on a Model A. Also make sure you have a good strong return spring.

Re: Disc brake question
 

Did you install a dual master cylinder when the disc were installed. If not the 10 psi residual valve that is built in to the stock master cylinder keeps enough pressure to cause the discs to drag.

Re: Disc brake question
 

Yes, by all means disc brakes use at most a 2lb. residual valve (or none at all). Generally if a higher value valve is used it will cause dragging.

Re: Disc brake question
 

Disc brakes don't have return springs.

Re: Disc brake question
 

When I put disc brakes (front only) on my 50 passenger car with original master cylinder under the floor the only change I made was to remove the residual valve from inside the Master Cylinder. This is the Jamco disc brake conversion kit and that was their instructions on what change to make with the MC. Been driving it for at least ten years like that and no problems. I changed to DOT 5 brake fluid about three years ago and still no problems. The braking is a major improvement over the drum system.

Re: Disc brake question
 

Adjust mc to pedal rod

Re: Disc brake question
 

I don't think a 10lb residual valve would cause the discs to lock up the wheels. A bit of drag, maybe, but not getting tighter and tighter.

An incorrectly adjusted push rod would cause the problem, maybe made worse by a 10lb residual valve.

The valve causes drag, things get hot, the fluid can't get back to the reservoir so it sets up a vicious circle and it gets tighter and hotter and tighter etc. etc.

Back off the pushrod just a touch and see if that helps. If you have a problem out on the road, just crack off a bleed nipple. That will help temporarily.

Re: Disc brake question
 

I think he means a pedal return spring..... Mark

Re: Disc brake question
 

Disc brakes don't have return springs. That's true. The pads rely on free play in the bearings to knock the pads back. Excessive free play can cause a problem because the pads have to move too far before clamping the disc.

I think the rubber seals may also play their part in bringing the pads off the disc a small distance, but I'm not sure.

The bit about excessive free play leading to pad knock back is right, though.

Re: Disc brake question
 

Brake pedals have return springs. I'm well aware of how disc brakes operate.

Re: Disc brake question
 

I use a 74Fford f150 MC, as this was the first year Fors had Disk brakes as an option, and they were not powered. This MC works quire well and I didn't have a proportioning valve.

Re: Disc brake question
 

Marko's problem is a common one with regard to swaps from drum to disc brakes. As mentioned by several posters, this issue has to do with not having enough free play in the rod between the brake pedal and master. When your foot is off the brake pedal, because the piston in the master isn't all the way back at its "starting" point, pressure is kept in the brake lines and the calipers drag more than they normally do.

Residual valves are another (similar) subject. They keep pressure in the lines so that brake fluid doesn't bleed back into the master.... kind of important
with drum brakes because otherwise you'd be left having to pump up the brake pedal to enjoy short pedal travel. Not something you want to need to do.
On disc applications you'll see residual valves mainly if the master is mounted at a weird level, like lower than the calipers.
GM used a quick take-up master and caliper design years ago to keep rotor dragging at a minimum for increased fuel economy reasons, but that's a whole 'nother story.

Re: Disc brake question
 

check the freeplay in the master cyl if you have a vacuum booster check for play between booster & master cyl if these are not right when the fluid warms up it cant return to the master cyl reservor thus aplying the brakes

Re: Disc brake question
 

The main purpose of drum brake residuals is to keep air from entering past the wheel cylinder cups during release, because of the strong shoe return springs. A side benefit is a lightly charged fluid line, minimizing pedal movement.

The 2lb disc residual is strictly aftermarket, and should only be used when the master cylinder is lower than calipers, to prevent fluid loss from the calipers. :)

Re: Disc brake question
 

I remember a similar problem, years ago, solved by bleeding the brakes. Seems a little air in the system heated up and expanded as the brakes were used and locked them up. Moved fine when they cooled down.