1994 ford ranger

not my truck but i'm asking for a friened her front brakes lock up i told her power booster or ppv. what do all you think? she don't trust anyone.

It could be the proportioning valve, but if that were the case I'd expect the rear brakes to lock up instead. There is a one way valve in the vacuum line that runs to the brake booster. You should be able to blow air through it towards the engine. If it blows both ways or not at all replace it. If the rubber brake lines running into the caliper are bad internally they may not release pressure to the calipers. The insides of the piston cylinders could be rusted or rough not allowing the piston to retract back. The calipers themselves may be loose, binding or sticking. I"d take the calipers off and check everything, then if it all looked OK I would replace the booster and check valve.