78 F250 brake problem

[redgeep87]

I'm a new owner of a 78 F250. I drove the truck home about 30 miles and the brakes worked fine. Being a 40 year old truck with dry rotted brake hoses and rusty brake lines I decided that the brake system need to be replaced. Everything was replaced except the vacuum booster. Bleed the system, I have a good pedal. The problem is while driving the pedal gets harder and harder. After about 5 miles the rear drums start sticking. Its like the system isn't letting the pressure off the rear drums. On the end of the proportioning valve there is a bolt with a nipple in the middle. Is this a adjustment for the movement of the internal valve. If not what could be the problem, and fix.

[colnago]

The only thing that comes to mind is that the pushrod between the booster and the MC might be adjusted too far out, so it's always putting pressure on the brakes. When I redid my brakes a few months ago, I bought a new booster, but didn't have the pushrod adjusted out enough, so even though the pressure was good, I had a loooooong pedal travel before I got anything. I think there should be 0.010" between the pushrod and the MC cup, which gives the pedal about 1/4" before it starts to apply the brakes (could be wrong on the numbers, so double-check).

Other than that, I dunno. Did you replace any of the lines? Are any of them running close to the engine or exhaust, and maybe getting the lines and brake fluid too hot? Do you have in-line pressure valves (sorry, don't know what they're called) going to the brakes? There are different pressure values; maybe you have the wrong one in the drum circuit.

Keep us posted.

Joseph

[redgeep87]

Thanks for the reply. I'll adjust the rod to see if that is the problem. Yes all of the lines were replaced. I have double checked for clearance thinking the same thing about heat.

[MadMike]

*Warning; I may go on a bit of a tangent*

Are any of them running close to the engine or exhaust, and maybe getting the lines and brake fluid too hot?

Brake fluid will expand, however with the system at an in rest state(foot off the brake pedal) and everything adjusted correctly, the fluid expanding will never cause the brakes to drag. If that were the case roadracers would constantly be battling such a phenomenon.

Not only will you want to check that the pushrod is correctly adjusted, but that the MC pistons are properly returning to their at rest state.
If the are and the problem persists it could be that the compensate port is blocked or plugged. This port allows excess fluid from expansion to return to the reservoir quickly, this ports also allows the brakes to be released quickly when your foot is removed from the pedal, as the normal feed port would take too long to properly release the brakes.

Do you have in-line pressure valves (sorry, don't know what they're called) going to the brakes? There are different pressure values; maybe you have the wrong one in the drum circuit.

Pressure residual valves, usually in 2lb(disc) or 10lb(drum) sizes.
These valves are usually only needed when a modern MC/reservoir is at the same height or below the calipers.

With older vehicles the return spring of the MC would actually create negative pressure that would overpower the 'rubber' cups on wheel cylinders. They were not reinforced like modern wheel cylinders. A residual valve installed into the MC would prevent the fluid from flowing back faster than the cups could maintain seal.

Wheel cylinders, particularly, can cause an air leak without a fluid leak. This would allow the cylinders to suck in air past the seal but not leak fluid back out. This can lead to a 'mystery' air leak and other PITA diagnostics while a 'mystery' of the BMC starts to appear to 'overfill' itself. Nope, its just basic self siphoning(on a low mounted MC) or reverse pumping the fluid to the reservoir.

By the late 60's early 70's residual valves in the BMC were starting to go away as the wheel cylinders themselves would now have a metal cup(expanders) to reinforce the 'rubber' cups that sealed the fluid in behind the metal pistons. No need to have residual valves to prevent the wheel cylinders from sucking air past the cups.

If you have a drum system that has a newer design/style or disc/disc BMC lacking residual valves and older style wheel cylinders lacking expander cups, the system will suck air through the wheel cylinder cups everytime the pedal returns to rest.