I've got a brake problem that is driving me crazy, any help would be greatly appreciated. It's an 85 4x4 pickup.

When I go to stop I have hardly any brakes until the pedal almost hits the floor and then the front tires lock up and skid. So first I bled the system, still had the same problem, bled it again including the load proportioning valve, I replaced the master cylinder, cleaned the rotors and sanded the pads, still the same problem. I checked the check valve for the booster and it functioned properly. All I can think of at this point is the booster; but I don't see how that would cause the problem because I don't have to use much pressure to lock up the brakes. I'd really appreciate any advice.

 

Have you checked the adjustment of the rear brakes?

 

I haven't, I was thinking that could be an issue. I'll look into that tomorrow. Maybe I'll just rebuild them in the process.

 

They most certainly can cause the problem your describing. Another sign of this is if your parking brake comes out real far and doesnt hold the vehicle well.

 

It does come out pretty far but does hold well. Thank's for the input. I'll definitely check that out tomorrow.

 

Also check the rear wheel cylinders. Sometimes they start to leak, but not to the ground. Pull back the boots and see if they are full of brake fluid. On most of the vehicles I have owned, those have been the first hydraulic parts of the brakes to fail.

 

The reason behind this is leak in the hydraulic system which is not allowing the brakes to hold pressure. Get it checked from Toyota mechanics only.

 

Your front brakes do 65% or more of the braking, so if you had zero rear brakes, you would not have "hardly any brakes". They'd feel weird and you'd nosedive more and they'd lock up prematurely, but not to the extreme you're describing.

I suspect stuck caliper pistons. How's the pedal feel?

 

Your failing to take account of all the fluid being used to expand the wheel cylinders on the never adjusted shoes .

combine that with worn shoes and drums it will behave just as the Op describes

 

Isn't the master cylinder dual-circuit? Otherwise it'd only have one output

 

You are correct. The master cylinders on these trucks are dual circuit, but when one circuit fails, it will completely compress one of the pistons in the master cylinder before the other one gets pressure, so the pedal will go close to the floor before the second can start to apply pressure to the other circuit. Older vehicles only had single piston master cylinders which would lose all brake function when one part of the hydraulic system failed.

 

I have had two faulty boosters. One of them would apply the brakes when driving down the road and me not touching the pedal. Will it roll freely going down the road at higher rpms? I would see if the rotors are hot when you are experiencing your symptoms.

 

I learned something new, thanks.