Rear brakes lock up

[caddman]

whenever I brake my back breaks lock up no matter how hard I apply the pedal.  is this a proportioning valve issue?
the breaks also grab hard right at first then back off.  I'm in a fix here, I just wrecked my motorcycle and need my car on the road ASAP.
any input is appreciated.

caddman

[resq302]

disc brakes or drum brakes or aftermarket set up?

[caddman]

front disk, rear drum.  completely factory with a vacuum canister add on.

caddman

[resq302]

I had the same problem when I got the rear "hold off" valve from Fine Lines.  The valve would let fluid through to the rear brakes but not let it return to the master cylinder.  When the rear brakes are locked up, jack up the car and go through and crack open the brake lines.  If you have fluid squirt out, the problem is forward of the area you cracked open.  If you crack open the line and it drips out, the problem is after that junction.  This is how I located my problem area.  Hope this helps.

[caddman]

they only lock up with my foot on the pedal.  they don't stay locked up, but there is no medium they are all or nothing.

[Musicman]

Could be a failed proportioning valve, but it sounds more like your shoes are not fully retracting for one reason or another.

[caddman]

i'm thinking that i'll try an adjustable proportioning valve and see if that will help me out.

caddman

[Musicman]

Are you stepping down normally on the pedal when they lock up.. (IE: their just locking up before the front) or are they locking up as soon as you touch the pedal (IE: little or no travel on the pedal - High Pedal)?

If the drums are locking up under normal breaking conditions, then the prop valve may be at fault. However, if they are overly sensitive and locking up with little or no pressure on the pedal (High Pedal) then I would look elsewhere.

[caddman]

they lockup with the lightest touch of the pedal.  it makes for a little tricky when stopping in traffic.

[acelondon]

proportioning valve

[resq302]

Another possibility too could be the rear axle seal leaking and contaminating your brake shoes.  Normally if the proportioning valve is going bad, the brake light would light up on the dash.  At least that is what mine did.

[caddman]

i've considered that.  I don't think I have a leaky axle seal but i did have a leaky wheel cylinder a while back so I think i'm going to just replace every thing on the back, parts are only about 120 for everything so figure i'll try that first.

caddman

[resq302]

yup, brake fluid on shoes and/or pads are just as bad.  You would think that gear oil would make the pad slip more and not grip more but it seems to grab better with the oil soaked into the shoe or pad.  Who knows.... im not a chemist.

[caddman]

yeah and I not positive but i'm pretty sure now that it is only one wheel locking up and its the one that had the bad wheel cylinder.  so that is where I am with it.  Now I just have to wait til I have the money to make the changes.  as far as the comment about the brake light, I had an electrical fire and had to rewire.  so I don't have a break light warning.  thanks for the input.

caddman

[resq302]

The brake light is a good thing to have in the even that you have a leak or sometype of pressure difference at the proportioning valve.  I'd recommend that you get that back in there when you can.

[Nacho-RT74]

clogged lines ( did it happen to me on a buddy 93 Buick lesabre front disc lines )

[John_Kunkel]

A proportioning valve doesn't go to work until the pressure is well above the pressure needed for a normal stop. (heavy braking brings in the prop valve)

I'd pull the drums and look for a mechanical cause for the premature lockup.

[Musicman]

A proportioning valve doesn't go to work until the pressure is well above the pressure needed for a normal stop. (heavy braking brings in the prop valve)

I'd pull the drums and look for a mechanical cause for the premature lockup.

Sorry, just getting back to this thread... John is correct, if the prop valve was not functioning, you could lock up, but you would be pressing pretty hard on the pedal before it happened. If your locking up just touching the brakes, then chances are the brake shoes are not retracting all the way back to the pin stop for one reason or another. Inspection time....

[caddman]

just wanted to update everyone.  I replaced everything out back, drums, shoes, springs, adjuster, wheel cylinders the whole nine yard.  the car stops like new! it was 200 bucks well spent.  in the 20 years i've owned the car it has never stopped so good.  next I'm replacing everything up front.  have a good holiday to everyone, and thanks for your imput!

caddman