Hydroboost brakes on 1988 chevy truck

all 4 brakes lock up --do you have rear drums --no abs activation ---is their an aftermarket brake controller installed --see the problem is that your abs will activate when a wheel starts to lock up so you should have some kind of code or trouble light --in comments if you could add additional information

I think that van has "hydroboost" brake assist instead of vacuum powered assist. Hydroboost uses pressure from the power steering pump to assist the brakes. However, I am unsure where the problem is. It could be a leak in the hydroboost itself, a leak in the line, a bad pump, or as easy as low power steering fluid.

Sounds like you have hydroboost, an alternative to vacuum booster for brakes. Hydraulic pressure from steering is used as driver assist for the brakes.
http://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/

You can read about it on the link. Hope it helps.

Always start back at the beginning. Remove the master cylinder and bleed it before fitting. Then you can bleed the hub furtherest away from the master cylinder, working your way towards the closest. Also make sure all the pistons/wheel cylinders are pushed all the way home first.

http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?pageId=0996b43f80c90f01Just click on link to see brake repair.

It would appear that your vehicle has a Hydroboost system (many Chevys do), which uses hydraulic pressure from the power steering pump to also boost the brakes. Check your power steer fluid level. If it is OK, then either you have air in the system, or the pump is failing. Also check all the hoses and pipes between the pump and brake master cylinder for leakage.

This sounds like the hydroboost unit is leaking.behind the master cylinderThe braking system on this using the power steering pump to provide power assisted brakes instead of a brake booster. The hydroboost unit most likely needs replaced.

THIS vehicle has a hydroboost brake booster which runs off power steering pump. is both went out you have a problem with the p/s pump

Sounds to me like the hydroboost system has a stuck biass valve in the hydrobooster. The sytem works both the brakes and the steering.

Check to see if the power steering reservoir is full and the belt is intact.I'm assuming that you have a hydroboost unit where the p/s works the brake booster. If those are ok you could have a bad p/s pump