master mods with rear discs?

[Nigel]

I did a rear disc conversion on my 70u a while ago and after some of trail time it is clear that the rear brakes are dragging pretty hard. I was told once (while drunk ) that there is a one way valve in the rear output from the master cylinder that should be removed. Is this true? I am going to strip down the brakes over the weekend as they need to be greased up after the fall mud and water.

anyone do any mods to their master?

[FordFascist]

Some master cylinders have a residual pressure valve built into the port for the rear brakes. It looks like a reed valve, basically like a little flap of brass behind the flare fitting that holds pressure in the line to fight the force of the drum brake return springs.

Most new ones however, have that feature built into the stock proportioning valve.

[Nigel]

mine is a '78 so there is nothing new about it.

So how does one remove this thing?

[VancoPBS]

Originally Posted by Nigel

mine is a '78 so there is nothing new about it.

So how does one remove this thing?

78 doesn't have it, it's in the prop valve. They stopped putting in residual valves on Ford trucks in 76.

[Apogee]

Originally Posted by Nigel

mine is a '78 so there is nothing new about it.

So how does one remove this thing?

Gut the proportioning valve. Since you're running 4-wheel discs, you don't need the metering orifice in the front of the combination block either. It was originally there to slow down the front discs so that the rear drums would activate slightly before the front discs.

Be aware that you may need to add an adjustable proportioning valve in the rear line depending on your MC/caliper combination.

[Nigel]

what is the concern with no prop valve? Is it the rear brakes being to strong or to weak?

[NetBSD]

i dont have a prop on my 78 f150 and still useing drums, i broke the lines on the valve and had to hook em up without it to get home, ever since i have had no problems and just left it that way, rear is a lil stronger but i like it that way, ive had my rear end swing round on me to many times

[Apogee]

Originally Posted by Nigel

what is the concern with no prop valve? Is it the rear brakes being to strong or to weak?

Too strong. The concern with no proportioning valve is that in a panic stop type situation your rear brakes will lock up before the fronts resulting in an oversteer condition where the back end will try to pass the front. The proportioning valve is there to reduce the rate of pressure increase to the rear brakes when you jump on them...

Ideally you want your fronts to lock up just before your rears, that way all you have to do is let off the brakes to regain control (understeer), assuming you haven't already hit the guardrail or whatever happens to be in front of you.

Keep in mind that all proportioning valves are a compromise from the factory. Due to all of the variables involved in determining it, I suggest an adjustable one so that you can tune it to suit your rig, driving style, traction, etc. Your base brake balance should be designed into your system with rotor diameter and caliper piston area...if these are not somewhat closely matched (typically rear braking torque is roughly 2/3 to 3/4 the front braking torque) than a proportioning valve can only do so much to help things. I believe they are limited to 57% of full line pressure when they are fully adjusted as far as they'll go the one way. The other way they'll go to 100% full line pressure and act like they're not even there.

Hope that helps.

[Nigel]

off topic but Ihave a front brake dragging hard. caplier slide smoothly but it seems to be holding residual pressure. related?

[Apogee]

Originally Posted by Nigel

off topic but Ihave a front brake dragging hard. caplier slide smoothly but it seems to be holding residual pressure. related?

Typically it ends up being old calipers with old seals and corrosion on the piston and caliper bore that causes the calipers to drag excessively.

If you had drums in front before, there would have been a residual pressure valve (10#) in the front line. Whether it's located in the MC, line, or combi-valve is anybody's guess. Residual valves are designed to counteract the spring force of the shoes so that they don't fully retract/compress the wheel cylinders, requiring excessive travel before engagement. This should cause both front disc brakes to drag excessively so I doubt it's what you've got going on.

Nevermind...'78 was disc.

[Nigel]

calipers are new also