brakes stopped working, what can be wrong?
 

on my way home last night my 72 gmc with disc and drums brakes wouldnt stop anymore. the pedal started to go all the way to the floor and if i pumped the pedal it made no difference and still went to the floor with barely any resistance. So i let it sit last night and checked it this morning. i checked the fluid and it is fine. then i did a vacuum leak test. i turned the engine on and let it idle for a minute or 2. then i shut it off. waited another minute and pressed the brakes pedal about 5 or 6 times. each time was the same and the pedal went to the floor. this is the procedure my manual said to do. so i guess there is a vacuum leak in the system. the hose for the power brake booster is perfectly fine. what can i check to fix this, what do u guys think the problem is? thanks

could it be the master cylinder bypassing inside? if the vacuum is fine I would crack a bleeder and see if the wheel cylinders are getting fluid.

Master cylinder leaking internally.

under 25 bucks at any parts store.

pedal to the floor with little to no braking is typically the master cylinder. Typically, if you pump it fast, it will pump up a little and slow you down. Went through it twice...

First on my '68, manual brakes, It would slow it down, a little. I intalled a new MC, and now when I stomp on the brakes, I can almost make the passenger hit his head on the dash... remember MANUAL brakes!

I went through the same thing on my '99 Suburban. The wife tells me that the brakes aren't working very good, so I go test it. Scared the crap out of me! Pedal hit the floor and I wasn't slowing down! Changed the MC and they work great.

Ditto, MC, $15.

Last time mine did that, both rear wheel cylinders were blown apart. Something to check, but most likely is the MC.

i just had that same problem, so i changed my master. went for a test, couldnt stop coming out of my driveway. bleed the front brakes and they still went to the floor. so then i changed all fluid front to rear with a niffty bleeder that hooks to your air compressor and sucked evey thing out. went down the road and was getting a 4 tire lock up.

Ever lose a master cylinder with the suicide single cylinder single line versions the pre 67 cars had? LOL, I had a 66 Malibu that did that and I about wrecked on the highway. Had to pump the pedal as fast and as hard as I could to stop, the piston had corroded enough to let some fluid go around it and would produce almost zero fluid pressure.