Holy smokes... I don't know if it's the tires, the wooden flatbed or the something wacky in the brakes. But my truck locks up the rear tires at the first sign of a panic stop. They are grabby as can be. The brakes have been checked and I had a leak back there, but even with that fixed, they are super touchy.

Add to that the all out howling sound that a 37" Baja TA makes sliding across asphalt and the folks around town think that there's a demon loose!

Any thoughts on what I might check on the brakes? Maybe a distribution block that was replaced with the wrong part or something?

Thanks and BTW... anyone want a set of Baja TAs? ha!

Shane

 

So the rears were checked, were the fronts?

 

Yeah, we checked everything and it all "looks" good. It stops well typically, but if you stop too fast it locks the rear wheels in a heartbeat. I can't imagine that the heavy duty wood and steel bed really weighs that much less than the steel bed, but it could very well I suppose.

 

Some things that would cause the rear wheels lock up:

The brakes are contaminated
The other brakes aren't working
The brakes are adjusted too tight
Weight is transfer during braking reduces traction on the rear
Air in the front lines
Front brakes not applying equally due to damaged lines or deteriorated hoses
Wheel cylinders sticking or seized
Backing plates grooved so the shoes hang up

 

And a defective proportioning valve.
Of course, your front brakes could be sticking too, but since you said these are drums they're not as likely to stick as a sliding disc caliper is I would not think.

I just don't think it's the fact of a flatbed. My rears lock up if you look at them wrong, and I've got a stock bed with a camper shell on top. Plus it's always loaded with crap.
Before I parked it I was having a real time figuring it out. Never actually did (yet) but the only thing that didn't get the once-over was the prop valve.
The front brakes were in great shape, but they were doing literally nothing. I don't know how long they were like this either, since the rear brakes alone were hauling the truck down like nobody's business. I thought I had full braking.
Until one wet day that is, crossing some railroad tracks and painted lines and could not stop because the rears simply locked up and skidded over everything. What a surprise!

Turns out my fronts were zero and the rears were doing 100% of the work. On dry pavement that was fine. But put the tires on anything slippery and you lost any brake control you thought you had.

Anyway, I've been working on other things and now that the brakes are old and the booster and master are going to the floor with no effort at all, it's going to be time to revisit the problem.
Sorry I don't have a difinitive answer, but the bottom line is that yes, the prop valve can cause this as well.

In the factory valve there is a "delay valve" which is the thing you're supposed to pull (or push in some cases) when bleeding the front brakes. It's there to delay the application of the front brakes so that the rears kick in first. Just slightly ahead of the fronts.
If it sticks closed, or the valve is otherwise clogged, the fronts don't get their pressure.

Check yours out and maybe even figure out a way to keep the delay valve pulled out, or pushed in as the case may be, while you drive it to see if things have gotten better.
They describe a special tool for the pull-type valves. But I don't know what you'd have to do for a push-type like on my '79 F350.

What year and model is your truck?

Good luck for sure. With that combination of tire size and brake type, you need all the help you can get!

Paul

 

He doesn't have a valve just a distribution block if he has front drums.

Another possibility the rear brakes may be a little on the loose side.

 

Hey guys, sorry... I should have included that information. I have a 1975 f250 4wd with the HD d44 up front. Drums all around.

This is a good place to start. It's a crazy deal. It's only the back tires that lock up as far as I can tell (by the way the truck tries to swap ends! ha!)

All the soft lines are new. The rear wheel cylinders are new. The linings are not. And the brakes have just been bled front and rear.

Maybe needs new linings? They are in good shape, but I think the truck sat more than it got driven before I bought it.

Shane

 

If the rear cylinders were leaking then fluid got on the linings and will cause them to lock up.
Change out the shoes and use brake cleaner on the drums.
I would also bleed the fronts to see if I had a good flow of fluid to show they should work.
Dave - - - -

 

And check the front wheel cylinders to make sure that both pistons in each wheel cylinder are free and working. Both rear wheel cylinders had their rearward pistons seized in the bores, on my truck.

 

Thanks guys! I will work on those things. I didnt think about the fluid causing them to lock up. Thats crazy.

Meangreen92..... are you a gomeangreener?

 

Don't think so. What does it signify? Although my high school's colors are green and white, and one of the old retired buses, which was used for special events, did have "Mean Green" written on the header above the windshields, for all of the passengers to see.

The screen-name derives from a Mustang GT I had.

 

Hah! Oops...
Yeah, I saw the drum part and apparently chose to ignore the facts.
Probably why I should put down the keyboard and step away from the computer after midnight.

Thanks.

Paul

 

The University or North Texas is the Mean Green... there is a football forum with a guy named MeanGreen92. ha!

 

I'm guilty of it too, I think I only read what I want to sometimes.

 

Agree with all the suggestions to check so far. With the heavier bed especially, rears should not lock up super early.

In the spirit simple things first, suggest you bleed the brakes (again probably), especially front, to make sure there is not some air in there slowing things down.

Also, as general comment, the OE braking system is asked to work a lot harder with big tires. Especially with drums up front. Front discs and maybe even hydroboost may help a lot.