ok, i have a 1994 silverado 4 wheel drive extend cab and i just did a complete rear brake change with shoes and cylinders and at the end of the test i blew out the left rear cylinder, not sure what happened.
Before the change the truck wouldnt move unless i stabbed the brakes and if i was on a decline it would roll. After the change the left rear piston in the cylinder actually blewout, fluid everywhere. The shoes were adjusted out to meet the drum and the pedal was high and firm.
My question is is there a residual valve in the brake line that holds pressure< could there be a reason the lines created a vacuum and sucked the piston in and upon applying the brakes pushed the piston out. Thank you in advance

 

Cant see it.....Usually when that happens, the shoes are not installed properly so that the plunger pops out when pedal pressed. If the shoes are adjusted out properly, the plunger in the wheel cylinder has no where to go. Not sure what happened in your case. Are the drums turned oversize?

 

drums were mic'd fine plenty within spec. I have read instances when the residual valve has failed and when it does it actually creates a suction when releasing the pedal and pulls the pistons in far enough for the pistons to push the seal out of place. Residual valve is in place on drum brakes to hold a certain amount of pressure on the shoes to the drum. Like i said i havent heard first hand of this happening but know its happened.

 

Hold on here, what is a "residual valve"? This term is not one that is used on any breaking system I know of ... So the way the rear drum breaks work, like poncho62 said, the wheel cylinder pistons push outwards with equal pressure moving the shoes outwards until they rub on the drum. When you release the petal, the spring inside the master cylinder moves the master cylinder piston back to resting position, causing a return effect on the wheel cylinders. The seal on the wheel cylinders that seals the piston to the housing, causes the piston to return with the action of the master cylinder.

There should not be any suction in the breaking system at all, unless the break pedal is released, and the calipers/wheel cylinders are returning. You bled the air from the system after you replaced the wheel cylinders, right? Then there is no residual pressure what so ever. If you put the truck in neutral, and aren't applying the breaks, the truck should roll. That's because there's no pressure in the breaking system to push the friction material onto the rotor/drum. If the truck doesn't roll, or it feels like the breaks are grabbing, then something else is going on.

It's also quite possible that you have a defective wheel cylinder.

 

The residual valve looks just like a union in the brake line, they are mostly found on drum brakes and they are designed to "trap" if you will the fluid pressure returning up line in the brake line so there is brake fluid pressure the next time the brakes are applied. Ive never seen them fail but have heard third party of people having issues with them.
If you think about it if you release brake pressure the pedal return can pull the fluid back towards the master cylinder which would create a suction pulling the pistons of the wheel cylinder to full in and the next time the pedal is depressed the piston could catch the seal upon travel out. Dont forget the shoes are adjusted out and held out by the self adjuster so if there is such issue with residual valve the piston will loose contact with the brake shoe. Like i said ive never witnessed it but know third party has had issues.
I removed 2 of the residual valves today just to clean and reassembled and double checked brake shoe assembly and found no issues as of todays test drive.

 

As I understand residual valves your rear brake drums only require one residual valve located at the master cylinder where your front disc brakes will need their own per side.

Your rear residual valves will need 10psi to counteract the pressure spring in the valve. There is a right way and a wrong way to install a residual valves..
Make sure the arrow is pointed in the right direction.

 

I've never experienced the wheel cylinders being pushed in too far to the point they catch the seals. As far as I know they have limited travel to prevent this. I'm not saying that the pistons cannot be pushed out from side to side, but that if you equally depress the pistons of the wheel cylinder, they bottom out on each other before any damage happens. If the pistons aren't pushed back into the cylinder equally, I can see how the seal might roll.

I'm guessing you just had a failed wheel cylinder. Seal just blew out.