Hello,

I have a 1993 F150 2WD, 302. The brakes are very sensitive. When I apply the brakes just a little too hard the front brakes lock up. This is brutal in winter weather. Over the past little while, I have become custom to applying the brakes just right, however if someone else were to drive the truck, I am sure they would end up through the windshield. I have been doing some reading and found that the cause may be the proportioning valve. Does this sound about right? If so, is it best to just replace the valve? Where would I find the valve, under the hood below the master cylinder, mounted to the firewall?

Is there a specific name for this valve, or is it just the Brake proportional valve?

Thanks

Derek

 

are the backs adjusted right?

is master cylinder full of fluid?

 

Yes, I have checked the back brakes, everything is good, and the master cylinder is topped right up.

 

clogged line, bad master cylinder, faulty caliper, incorrectly installed calipers/pads.

all things i would look at.

also the vacuum booster may be the problem.

 

Pads and calipers are installed correctly. I did the install. LOL.
I inspected the calipers and they are stroking properly.

how can I test the vacuum booster?

So is sound like the chances of the proportional valve failing is slim.

 

If it were calipers, it would pull to one side. If it doesn't do that, we can rule that out.

It could be a clogged rear line or ABS valve. If you take the drums off and step on the pedal, do the shoes move out? (don't go too far, or you'll blow the wheel cylinder apart)

Master cylinder is unlikely, and we can rule out the booster altogether.

I would have to say it's either the ABS valve, the prop valve, or the pushrod is adjusted too far. There's an adjustment procedure in the haynes manual I think. Lots of times when people replace the booster they don't do it right and you get grabby brakes.

 

This is very often caused simply by dirty shoes/pads. Make sure there is no oil or other contaminants on them. Just a little bit of oil will cause them to grab like crazy...

 

The proportioning valve only affects the rear brakes, because they being drum brakes tend to lock up when given identical pressure to the front brakes by the nature of the way the shoes grab.

There is no proportioning for the front brakes.

Usually disk brakes grabbing is caused by being dirty or rusting. Sometimes you can get a set of weird discs that corrode overnight and the first few stops in the morning are grabby.

If this is a constant thing, and the discs are clean, then I would suspect the booster,.......but I've never experienced it myself personally so I don't really know how a defective booster may feel.

 

I have checked the rear cylinders by doing what you point out (remove drum and press brake. Both all shoes move.

So you are saying that the push rod on the master cylinder may need adjusting?

 

Brakes are new and clean and the problem happens all the time.

 

A defective booster would make the pedal rock hard usually. check and make sure none of the brake hoses are twisted on the calipers, and that your pins or slides are free and not sticking.

 

Did you use metalized pads? If so, did you have the glaze removed from the rotor (turned them)? This is an inherent problem with metalized pads on a used (unturned)rotor. My OE pads were doing the same thing at 130K on them. I put on "mineral" pads, and it ain't as bad, but didn't have the rotors turned, and it tends to still grab a bit. But yeah, any wet weather, I had to feather them lightly until I heated them up a couple times, unless I got up to a decent speed so the inertia of the truck would burn them clean again before they could lock the wheel. I hate the new pads because I now get horrendous amounts of brake dust from them on my chrome wheels, something the OE pads NEVER did. Rear shoes are still OE at 155K. Gotta love the M5OD for a brake saver. I've heard some really nasty things about ceramic pads unless you replace the rotors with some rather expensive ones designed for use with ceramics. Take the rotors and have them turned, if only enough a couple mils to cut the glaze off them and true them. It's very possible they're not completely true any more, and even a minor high spot will drastically affect braking. It'll seat the pads quicker too. EVERY brake shop around here will tell you don't bother putting on pads if you don't turn the rotors. Pads tend to soak up moisture in damp weather, and will grab like crazy on a really smooth (read worn) rotor.