My rear wheels lock up under moderate braking. When I bought this truck, the brakes were pretty much nonexistant. It was hissing through the firewall, so I replaced the booster, and the master had leaked out into the old booster and filled it half way up, so I replaced the master cylinder as well. Then when I went to bleed the system, I found that the rear wheel cylinders' bleeders wouldn't open, and even if they would the cylinders were siezed right up, so today I put new wheel cylinders, shoes and spring kits in the rear. So after all this, I bleed the brakes once again, starting from the farthest away. Now I have plenty of pedal, but the rear wheels lock up when applying moderate pressure to the brake pedal. I am thinking one of two things: Could the condition of the front brakes have an effect on the rears?, the rotors warm up after a 5-10 minute ride so I think they are OK, but... Or could the proportioning valve be bad? The latter seems more likely to me because about the time I bought it, it also blew a front brake hose, which I replaced after about a 20 mile trip home with it, but any ideas are welcome. I just want good brakes!!! Thanks!

 

mine has had this problem since i have owned it, i always assumed this was normal with these older trucks, but maybe not. i would like to know if this isn't normal too, and what it takes to fix it. sure makes for some interesting emergancy stops! (usually end up in the ditch or median!)

 

Blowing a front hose or bleeding the brakes improperly can result in the proportioning valve shifting to cut off the front brakes (safety system).

It is NOT normal to have the rears lock up!

 

oh, well maybe i should check that! what should i check?

 

Needlesss to say the rear end of pickup trucks are light, when compared to the front, and on older trucks like ours the rear brakes tend to lock the wheels before the front. Put a load in the back (800 lbs.) and you will see the difference. The proportioner valve is mounted on the inside of the left frame rail almost directly below the master cylinder. It is possible to remove it from the truck, partially disassemble it, and make sure it's working properly. Remember that this item is over 25 years old and has probably accumulated rust and garbage on the insides. I've had mine apart twice in the last 10 months to clean it up and get it working again. The last time was for my breaks not fully releasing. I called my local Ford dealer and lo and behold, it is still available from Ford. It is a bit pricey at $140.00 but when balanced against properly working brakes I guess it's not too bad. *** Truck out of Lenexa, Kansas wants $169.95 plus shipping and handeling. I ordered a new one from Ford, should be here today. There is a proper proceedure for bleeding the brakes which involves extending the pin on the front of this valve that should be adhered to in order to properly get fluid to the front calipers. Hope this helps.

 

If I am getting fluid from both front and rear wheel cylinders while bleeding the brakes, would this mean that my proportioning valve is not causing the problem? Or could it still not be completley centered, letting more pressure go to the backs than the fronts?

 

Proportioning valves are self-centering. So, if they are improperly functioning, then either a disassembly/repair or just get a new one is in order. Motorhead, I would definitely start with the valve. If you've done the shoes, spring set, and wheel cylinders, not much else left. Are you getting good flow at the rear? Did you get the drums turned when you replaced the shoes?

 

Grape:
Yes, I am getting good flow at the rears, at the fronts as well. I didn't have the drums turned or replace them, they probably could use it but they weren't severely worn or damaged.

I didn't replace the calipers or pads in the front, the rotors look good, and the pads have some life left on them. The rotors are hot after a trip through town so I think the fronts are alright. Its just the darn rears locking up, usually while braking for a stop sign, and especially while braking on a downhill slope. They will lock up really easy off the pavement.

I will probable take off the prop. valve early next week (gonna be out of town this weekend). How far can they be dissassembled? right to the slider valve inside or...? What should I look for in there? I did notice that the little button on the front of it (the one people talk about pulling out) is broken off, but I made sure it was pushed in while its driving.

Thanks again,
Dan

 

Another Dan,, oh boy. But anyway,, check for any sign of marring or scratching of the walls/cylinder. Anything that would keep the valve from centering all the way. Seems like it doesn't want to release from the back all the way. If you could, check one thing for me, take the calipers off and make sure you have good movement on the piston. Should slide in and out nice and easy. If they are sludgey or seizing up, then more pressure would be being applied to the back, thus the locking. This is only if the valve is faulty though. If it's a good valve, I believe, they should work fine independently, hopefully someone else will have some input on this part. That pin that is broken off is for the bleeding procedure, to allow the fluid to flow to the front wheels. With the truck at a standstill, no brake applied, the front wheels are closed off. Then open after the back ones. That's the job of the valve, starts braking in the back and then transfers the pressure to the front as momentum shifts forward. I'm not sure I'm making complete sense here, so ask if anything is unclear

 

Grape Thanks for the input. I won't get a chance to tear at it this weekend, but next week for sure. You are saying I should check the calipers for stickiness before I pull the prop valve out, right? THat would make sense to me.

I will let ya'll know what happens when I check those two things out.

Dan

 

Yeah, might as well at this point, the calipers are alot easier to check for smooth movement than pulling the prop valve out, let alone tearing it apart. The last time I replaced one, on my old '73 F-100, I just went to a junyard and found a truck with a decent lookign on that had clear fluid still. Worked perfectly and a hell of a lot cheaper than a new one. I'd rather buy 10 at $10 apiece than pay the dealer cost. Thing is, when you bet around to taking it apart, just be real careful of dinging or scratching it.

 

Did you repace the shoes? If so, are they on correctly?

There are two different lengths of lining, you want the longer lining to the rear and the shorter to the front. Having them on wrong could cause the rear brakes to lock up.

 

You guys might be looking too hard. I have had several vehicles do this very same thing, and all I had to do was replace the axle seals. Contaminated brakes have a way of locking up. Just a thought, Ted

 

Yes, I did put the longer shoes on the back. As far as the axle seals, they don't appear to be leaking. Also, the rears grabbed immediately the first time I used the brakes after installing them. I will get to looking at the fronts and the prop. valve as soon as I get time.

Thanks