gmc 94 yukon brakes

[poppa]

I didn't get to look at it good yet , but my brake pedal goes to the floor. I peeked under and saw no "wet" wheels , dripping or anything else. Tues night I had to slam on the brakes (freeway) ,Thurs afternoon brakes went to floor. No brake light . Master cylinder? Will need soon , 8 inches coming.Thanks

[A383Wing]

If the master cylinder is full, and there are no leaks anywhere, sounds like the master is bypassing internally, time for a new one       

[poppa]

That's what I was thinking. I'll be on it first thing in the morning. Thanks for the reply

[birdsandbees]

I had that happen on a GMC and thought it was the hydrobooster system on the vacuum booster / master cylinder. I even bought one and thankfully they took it back as it never came out of the box. Front left brake line had a hole in it... and it shot straight out the wheel well at cars beside it!   It did not leak sitting still and I may not have caught it if not for the guy beside me at the lights screaming at me..  

When I replaced that line I had good brake pressure for about 3 pumps... and then blew the other old lines. Ended up doing all 5 on the truck.

[poppa]

Still see nothing wet , rear sump very low , almost empty. Brakes pump up nice with out it running , mushy when running. How can I test booster?? Tried handheld mighty vac ,won't hold a vacuum. Tried small vacuum pump , got to maybe 20 ,as soon as I shut it off it went to zero.

[poppa]

No fluid between master and booster. Unplug abs and pedal seems good?

[birdsandbees]

AHHHH.. the old front wheel sensor gig! When GM's front wheel sensors fail, they think the wheel is locked up so release ALL braking!! LEave the ABS fuse out until you can diagnose which hub sensor it is. Google GMC abs wheel sensor... apparently many times they just need to be cleaned and reinstalled.

[poppa]

Thanks , I'll try that

[birdsandbees]

It's actually probably BOTH fronts have gone... that's what happened with my '98 and almost rear ended someone, had to throw it in reverse.

You probably had one not working and now the second has gone..

[poppa]

Did the (sensors) r & r , cleaned them (some rust fragments) seems ok now. Thanks all , and Merry Xmas!

[birdsandbees]

Glad to hear that worked... the "free" fix!   

[poppa]

Always the best kind