Locking up the brakes

[mcarlson11]

So the other day I decided to lock up the brakes on purpose just for the hell of it. I have discs in front with drum rears. I have the correct proportioning valve and a dual MC, everything for disc swap came off one truck.


Anyhow, my rear drums locked up before I could even get the front discs to lock up. Is there a way to adjust the proportioning valve to correct for this?

My valve has no wires, because they were clipped right at the valve, I cant even splice in wire. So I cannot use a light to set this thing.

Any Ideas?

[Wes]

was the bed empty ? w/o weight on the rear axles there isn't enough friction to keep the tires rolling, so you get lock early.
Hope this helps
Oh and you can get the proportion valve plug and wire from the pick and pull. If you have pressure front and rear the valve is "set". There is no adjustment on the factory valves, only after market HP units.

[averagef250]

Try taking it apart and cleaning it. Every old 1970's prop valve I get is half full of rusty crud. They're really simple inside. Just some springs and small pieces that zing under your bench. O-rings and crush washers seal them, stuff you can usually re-use.

Even empty, the fronts should lock right away in a panic stop.

I had something similiar happen after swapping a 3.5" brake dana 70 under a 250. I expected the brakes to lock early, but they were extremely touchy. Turned out the rear had some sort of severe duty ceramic shoes. Put standard ones on and all was well.

[heep70]

Every old rig I have had has had the same effect with an empty bed.

[FORDification]

Are you sure you have a disc/drum master cylinder? If you've got a drum/drum version instead, it'll definitely lock up the rears way before the front.

If you do have the disc/drum MC, you can always get an aftermarket adjustable valve (from Wilwood, for example), which connects in the rear line between the factory proportioning valve and the rear brakes, which allows you to adjust the pressure to the rear brakes, to balance them more with the front.

[mcarlson11]

yes I am 100% sure that it is a disc/drum MC. However, I did not know that with an empty bed that the rears will lock up first. But since it is always going to be empty, I will look into a wilwood valve.

[averagef250]

BTW, I didn't mean it to sound like they should lock at the same time, just that the fronts should lock too.

Another cool option- All kinds of trucks use varaible rate proportioning valves mounted on the rear axle. Toyotas use them, dodge cummins truck use them even 1980's ford vans used them sometimes. You can mount one of those and bend the linkage rod to whatever setting you prefer. That way when you do haul something you can still stop!

[heep70]

You want one? I think I still have my old rear proportioning valve and adjusting rod off my 80 Toyota.

[Leadfoot]

if you got the wrong master or sticky calipers, old crusty hoses , even a oil seal leak the rear it likely.