Brakes on an '88 Toyota Pick up, why?

Possible bad brake booster. Try disconnecting vacuum line to booster (and plug it) and try it then. Remember brakes will not be power assisted, you will really need to step on them to stop. If it still dose it you may need ; master cylinder , brake hoses , ect.

If there weren't any chance that the wrong fluid was introduced into the system I would say it is the Brake booster. That was what I thought of first when I read your question. I have the '88 4X4 and it was the booster on my truck that caused the "brakes locking up" kind of thing to happen. But start with the cheap fixes first maybe. Drain the master down low and fill up with new fluid and pressure bleed that through the system. Keep adding fluid to the master until it comes out clear at the caliper bleed ports. The only problem with this is that if the absolute wrong fluid was added it could be possible all your seals and o-rings are going to be toast eventually. Perhaps catching this condition early will have saved them, if this was the cause. You can do the booster replacement too it is a fiddle but not a complicated job.

First question is: (sorry but you wouldn't believe how often this happens!) did anybody like some jeffy lube mess with your brakes? They could've - again, it DOES happen - added trans fluid or something like that instead of brake fluid. Actually, incompatible brake fluid would cause the same effect.

This would cause seals to swell and pistons to stick. Another - much less likely - possibility is clogged relief port in the master cylinder.

try cleaning the sides of the pistons in the front calipers. They do have boots on them but dust still gets behind them and causes EXACTLY what you are describing. Clean the sides of the pistons very well and the boots inside and out as well... I think that will solve your problem

sounds very unusual Ive never come across anything like this befor, but I had a thought that might be worth checking out, that is ; maybe your wheel bearings have collapsed and the brakes are holding the weight of the wheels. Dont know but its worth a look. Mirko.

was fluid added to the master cylynder? if it was, was it the correct fluid for your P/U? when you replaced front pads did you clean and lube all slide and mating points? first thing I'd do here is flush the brake system as best I could

with a truck that old have you rebuilt the calipers??? they can get very sticky and would not go back in