Locked brakes

[MotorPro]

On the return of our last trip we were almost home. I noticed the brakes getting spongy. After we got home I looked underneath and there was brake fluid coming out from between the master cylinder and hydroboost. Checked the fluid level and it was almost empty. I replaced the master cylinder and bled them until I got nice clean fluid out. Took it for about a 5 mile test drive. Brakes were better then ever. Last Friday I went out in the morning to fuel up about a  4 miles round trip. Everything was great. After work off we went heading for the camp ground. About a mile up the road it starts to slow down so I turned on a side street.at which point it came to complete stop. It would shift into reverse or forward and you fel it try to move but it was locked up solid. The engine tries to rev but is under a huge load. I tried several thing but decided I had better call good Sam . After a really frustrating hour on the phone and finding out that they are useless .ii starting it back up and away it went. I headed back home about half way there it did it again. Let it sit for an hour and this time made it home. The motorhome is an 86 elandan with 4 wheel disks and hydroboost. Anyone have a clue what could cause this?

[M & J]

Did you notice if any of the wheels/discs were smoking or extremely hot?
This has the classic signs of a collapsed flex line on at least one caliper. If it wasnt trying to pull to one side I'd say the rear. One line feeds both sides of the rear axle.

[Froggy1936]

There is a necc clearance between the master cly piston and the servo push rod , Did you check this when you replaced the master cly ?  It sounds like the system can not relax and its holding the brakes on !  If this is your problem , When the brakes are locked up , open the line fittings where they go into the master cly if that frees the brakes , You will have to remove the master cly and adj the push rod length!  Frank

[MotorPro]

No pulling to the side. No smoke from wheels. Didn't have temp gun with me to check temps. Can move up and down driveway over and over with no problem. I'll pull the master and check for clearance.

[Lukewarm256]

Sounds like you have solid advice to follow already. So I will throw random things at you to think about.

First, with my 68, when I replaced the master cylinder and bled the system, it  pushed a ton of sand and rust through the lines that collected in the wheel cylinders (callipers for you). Clear fluid, but packed with crud that wouldn't let the brakes fully retract. It's a smallllllll possibility.

Mix brake fluids? Some people say that some brake fluids don't mix (dot 3 with synthetic dot 5) and can turn into a substance that resembles paste. Once it's hot, it won't flow back out and let the brakes release. Possible?

Have not noticed any shakes or vibrations? Sometimes bad wheel bearings can make disk brakes hot and grabby.

And only heard of this last one once, on rear disk brakes, so it won't be it, but I'll throw it out there. Once upon a time, at work, one of the trucks had the clip holding the Axel shafts in place come out, and the only thing holding it all together were the brakes. It ran and drove, but it made the brakes lock up really fast.

Probably one of the much simpler suggestions above mine though.

[MotorPro]

Thanks for the suggestions.

[DaveVA78Chieftain]

Motorpro,
Next time be sure to open your thread on the Chevy board.  This area is sort of hidden to most users and your post can be easily missed.