I have a 1982 Toyota 1 ton RV, here is the problem, The front brakes lock up (Both sides) once the fluid heats up. Regardless of what is done. Here are the things that I have done : New front Brake Calipers (both sides), replaced rear wheel Calendars, Replaced front pads, replaced master cyl ( 3-4) times- not new, but rebuilt, changed fluid 2-3 times, checked brake pedal adjustment, dismantled and cleaned the proportioning valve. The brakes do work fine until fluid warms up. Once the fluid warms up, the front brakes lock and will release if I crack loose the front line at the master and if I crack loose the rear line at the proportioning valve (in effect, that's cracking it at the rear of the master). Also replaced two front rubber brake lines at the calipers. Every thing is new except for the proportioning valve (thought I'd mention that I cannot buy new, let alone even find), and the rear drum brake shoes (witch are adjusted and in good shape. What the hell is up?? If your info fixes it, I'll donate more.
If you crack the line at rear of master where does that line go? Does it go to rear of truck or front. If it is a drum type master there is a blocker in there to keep pressure on the drum style brakes. Drum styl ebrakes need this to operate to have a little pressure going to it, disc do not that may be the reason for locking up. If it's a proportioning valve you can always get a self adjustable one as longas it has the correct fittings for it like so many custom cars have. Not being able to see the truck this is about all the info I can give. There may be a metering valve for the front disc on the crossmember/axle someplace or it's tied in with the pro valve. You don't have any compression fitting s along your brake line somewhere as this wold cause what you are having. If you do remove them an dreplace the whole line or install a fitting on the line with a connector instead of a compression fitting. One other thing recheck your parking brake to make sure it is not keeping your regular brakes applied all thetime. Do the calipers have new kits on them and slide well inside there mounts?
December, 10, 2012 AT 7:17 PM
Check the brake booster push rod to master cylinder measurements. When problem occurs, loosen the master cylinder bolts to test if pressure releases.
December, 10, 2012 AT 8:01 PM
Oneother thing, i'm sending a picture of a load sensing proportioning valve which your truck has, if you have disassemled this, it was NOT meant to be and controlspressures sent to fornt and back wheels. It is connected by back axle. If you have the pressures are screwed upand will need to be replaced as it can be adjusted but you need several gauges to get it. Best thing if this is what is wrong besides what KH suggested because I forgot about that my self, is try www. Junkyardog. Com and type in info. If they have it they can send it to you.