brake check valve?
 

93 b2600 2wd 5spd. I have come across this reference to a check valve in the braking system but cannot find anything except for emissions stuff. Can anyone clarify this for me? I have been wondering about this as a possible cause for losing pressure when pressing brake petal for booster testing. It builds up pressure then slowly releases it. While driving it feels a little soft. I have not bled brakes but plan on doing that tomorrow.

There will be a proportioning valve that splits the pressure front to rear. There may also be load proportioning valve on the rear axle to adjust brake bias from no weight to fully loaded tray.

A full clean & adjustment of the rear brake shoes & a full brake fluid flush is in order.

There is a check valve inside the short curved vacuum hose that leads right into the brake power booster. Is that what you mean?

I wasn't aware there was one in the hose. How do i test it?

Try to blow air through it; should only go in one direction. There is an arrow on the outside of the hose and states "to engine".

I haven't had one of these check valves go bad myself, but I've owned my '88 B2200 21 years now and have replaced the power brake booster exactly once. I have replaced the brake master cylinder several times over that time, and the calipers once each. I don't remember doing either rear wheel cylinder.

I'll check it out as soon as everything defrosts. We just had a small winter ice storm blow thru and everything is frozen till at least monday.