%^&*$# brakes!

[crazy longhorn]

I have done a bunch of brake work in my time, but this one had me swearin. I started to do a $14 set of pads on the old 78 K 20, & ran into a collapsed hose . well, 2 hoses, & 2 calipers later & a bleed job , i am back in brakes. a few trips up/down the driveway, the peddle goes to the floor....WTF! i check the master for leaks, & its full....stamp the peddle HARD 4 or 5 times, & I have brakes again(for a little bit)....peddle goes back to the floor. the kid came out & we tried bleeding the brakes again, with no luck. my 1st thought would be master cyl......but i really wonder about that prop valve? We didnt get any air on the 2nd bleed, but not much fluid either . im up for ideas .....crazyL

[JimKshortstep4x4]

The prop valve cannot cause a loss of brakes unless it has a leak which is highly unlikely. I would lean toward the master cylinder if there are no leaks in the lines and wheel cylinders.

With what you have checked so far it really sounds like a master cylinder problem.

Jim

[lux hauler]

I HAVE seen the prop valve cause a loss of brakes. The valve itself (internal) was stuck....either too far forward or too far back (can't remember which). This was a new prop valve. Tried re-centering the valve per manufacturers instructions but it didn't work......they had been having problems with them. They sent a new one out. Replaced it and had brakes. What was different was this car had almost no peddle at all.

[franko72]

check the seal mating surface of the master cylinder cover gasket . be sure that it is clean at the casting surface and the rubber surface. had a similar problem as you, brakes going to the floor and the light coming on. I noticed a little surface rust on the mating surface, used some sand cloth on the cast surface and removed the cover gasket and cleaned it well . I reinstalled it and that solved my deal.I know that the reservoir is not under pressure , but it solved my problem.

[crazy longhorn]

Thanks guys, it looks like no leaks in the system(master still full), i did try the scuff the top.....looks like a new master is next. the damn thing just went from brakes to none, no spongy peddle or other warning! I 1st thought it blew a line , but no leaks. Oh well, better in my driveway than out on the street.....crazyL

[tmcgrawlookalike]

Originally Posted by crazy longhorn

Thanks guys, it looks like no leaks in the system(master still full), i did try the scuff the top.....looks like a new master is next. the damn thing just went from brakes to none, no spongy peddle or other warning! I 1st thought it blew a line , but no leaks. Oh well, better in my driveway than out on the street.....crazyL

sounds to me that the m/c is bad.just remember,it`s what creates the pressure in the system.probably the piston seals in cylinder are just getting a little tired!!!!!

[chevymad]

Since you just did brakes, if you pushed the pedal further down then you do when you normally stop, you can ruin the master cylinder. It pushes the rubber cup down into a part of the cylinder that hasnt had any wear. Theres usually a nice buildup of rust and debris down there. This either tears the cup or the debris just causes it not to seal. Then you have no brakes and you wonder why it quit.

[crazy longhorn]

Got it! Yup the rebuilt master did the trick. while I was into it, I popped the rear hub /drum assys, & installed a good used set of adjusters, & got the shoes adjusted up.....Got brakes now The old truck got a fresh set of plugs, tweeked the timing, & took her down to the shop & installed an H pipe. The H really tamed out those flowmasters.....ready to abuse the old ugly truck some more crazyL