brake pedal to the floor.

[Prerunner1982]

Ok..so my wife said the brake pedal went to the floor on her way home on Friday. Luckily she was within a couple miles of the house and made it safely.

The front pads were beyond worn so I changed them. The master cylinder was VERY low so I filled it up. I bled all the brakes. With the truck OFF the pedal will build and hold pressure, however with the truck ON the pedal will go right to the floor. The master cylinder is holding fluid.

What am I missing here? What broke? Booster?....

Help me out...please..

[FLEET72]

If you are not losing fluid and there is no air in the system then it's an internal leak in the master cylinder. Bad piston/seal.

[Prerunner1982]

After discussing the problem with my father in law that is what we came up with as well.

With the vehicle off not enough pressure is provided to force fluid past the seals.

With the vehicle on, and with the assistance of the booster there is enough pressure to force the fluid past the seals.

Once I thought about it I realized with the vehicle on the master cylinder sounded like it was aggitating the fluid rather than compressing it. I did not hear this sound with the vehicle off.

It makes sense... Thanks.

[Prerunner1982]

Ok, well I replaced the master cylinder and flushed the lines with all new fluid. Still having the same problems...

[jayharrell]

If the rear master cylinder reservoir is the one low, you probably have a bad rear brake cylinder. The rear passenger is usually the one that blows a seal. Just pop off the rear drum and see if it has fluid all over it. It will be obvious. You can probably pick up one of those cylinders for under $20.

If thats not the case, you probably have a front line leak. I've done it my self, change the brakes and get a twist in the line, first turn rubs the line on the frame and whamo, no brakes.

[Prerunner1982]

I already check the rear brakes cylinders and neither is/was leaking. I am not loosing fluid at all.

[jayharrell]

http://www.classicperform.com/TechBo...tm#testbooster

This link helped alot. Look at booster push rod lenght. After i replaced my master cyclinder i had to adjust about 3/8'' outa the push rod for the new one to work right. Easy wasy is to put the push rod in the booster with no MC, mark the rod with a marker where the bottom of the MC would be when in stalled, then put it in the MC and see if it lines up. Better to be too short then too long there. Too long won't let the rear piston come up to a full pump.

[68C15]

if this has ABS you will need to do the 1 1/2 hour long process of bleeding the WHOLE system once it gets air in it. NOT FUN!!

[Prerunner1982]

Yeah it has ABS.... the manual doesnt specify how to bleed the brakes with ABS. So I am currently trying to find the proper procedures for doing this.

[FLEET72]

Originally Posted by Prerunner1982

Yeah it has ABS.... the manual doesnt specify how to bleed the brakes with ABS. So I am currently trying to find the proper procedures for doing this.

If it is a factory manual it does.

[2manyprojects]

Originally Posted by 68C15

if this has ABS you will need to do the 1 1/2 hour long process of bleeding the WHOLE system once it gets air in it. NOT FUN!!

It sounds like you have ABS issues to me also. Thats a pretty common problem. It takes forever, it being a long process is no exageration. Try this. Grab a friend, relative, or trunk monkey who ever is available. Cycle the key on and off, waiting about 10 seconds in between each, 6 to 8 times. the more the better. Open the bleeder screws, PUSH the fluid VERY SLOWLY through the lines. Bleed like you normaly would opening and closing the bleeders to avoid getting air in the system. DO NOT PUMP THE PEDAL AT ALL AT ANYTIME and keep the reservoir full. Your supposed to wait a minute in between pushes, but I've been able to do it waiting 15-20 sec in between. To check you should be able to hit it once and have a hard pedal. It might take 50 or more strokes of the pedal to get it right. So dont be expecting a good pedal after a few strokes of the pedal. You might get lucky but I doubt it. It's been awhile since Ive had to do it, but that should be everything. Oh and if I didnt mention, DO NOT PUMP THE PEDAL lol.

[Prerunner1982]

Alright... Since I now have 2 weeks off due to a new baby I will give this a shot. I hate buying special tools though...

[89 crew]

If the rear brakes are drums check the adjustment. If they are loose it will take a lot of fluid to make them touch, then proportioning valve shifts and you are on the floor.

[Prerunner1982]

may I ask what the cycling of the key does?

[2manyprojects]

Cycling the key centers a spool in the ABS unit. What happens is the spool shifts to one side and blocks off flow to the rear wheels. Thats why you have a low pedal. It's something like that. It's been so long since I took the class I dont remember exactly. If you want exact details ,I might be able to find the book, somewhere around here. If someone else can give a better explanation feel free.

[Prerunner1982]

I got tired of looking at it and decided to limp it into town to a mechanic with the right tools. When I started it up and pulled out of the driveway the pedal felt stiffer than it had since I started dealing with this problem. Drove into town just fine, stopping as normal. I went ahead and had it bled anyway since it is my wife's vehicle and she will be driving around alot with our two daughters. The shop said there was still quite a bit of air in the system.. works fine now. I was pleasantly suprised that there was any pedal pressure today... but am glad there was.. Took some weight off my shoulders.