Brake pedal goes to floor

'97 Chev CK1500 stops fine, has a little too much pedal travel (in my opinion), but if you keep pressure on the pedal, it goes all the way down. 'Mechanic told me that was normal, and that he would pass it on the safety inspection! Something’s wrong! what is it?

That’s a classic symtom of a brake master cylinder that’s leaking internally. Replace the master cylinder, bleed the brake system and that should restore a firm brake pedal.


…and get a new mechanic - immediately! That one apparently doesn’t know that dead customer’s can’t pay their bills.

Agree with Tester - classic symptom of a failing master cylinder.

Agree with cigroller. A malfunctioning brake system cannot be ignored.

You’re absolutely right, except I’ve done the tests for internal leak and it checks out fine. I also took it to a mechanic close by and he drove it, said he stopped about 20 times, and it seemed fine. I showed him how the pedal will fall away (with the motor running), and he said that’s normal, they will all do that to some extent depending on the vehicle, and said he would pass it on the safety part of the inspection.

I’ve done the tests for internal leak and it checks out fine.

How did you test for an internal leak?

I ask because the issue is this: When you apply pedal pressure, the two rubber plungers/pistons are no longer making a good seal. Instead of holding fluid pressure in front of them, they’re allowing fluid to seep by them. Hence, you pedal slowly goes the floor.

Whenever you release the pedal, the fluid behind the pistons always migrates to the front of the piston (as part of normal operation). So how you can detect fluid seeping past the piston/plunger seal when the pedeal is pushed is mystery to me.

You test it without the motor running. Push the pedal down to relieve the vacuum and then pump it 5 more times and then apply constant pressure. If the pedal “falls away”, then it’s by-passing, or has an internal leak, if the pedal stays up, there’s no internal leak.

I need to respectfully disagree with your test method, which is incorrectly leading you to the wrong conclusion.

The problem your experiencing occurs with normal pedal pressure (where normal includes the vacuum assist). When you remove the vacuum assist, you’re reducing the pedal pressure to levels that are insufficient to stop the car.

The fact that you’re not seeing the problem without vacuum assist doesn’t mean the problem isn’t there. It also tells you nothing about the master cylinder, which based on your problem description, is still defective.

If this were my car, I would get the master cylinder fixed immediately.

The truck will stop with the vacuum booster disconnected. If it wouldn’t, it wouldn’t be safe. It just takes more pressure. This testing method is the same as bench testing, which is what they do at the factory and rebuilders. In my reply to another person here, I mentioned that including the master cylinder that was on the truck when I took possession of it (it belongs to someone else), it has had 5 in two weeks. The first 2 were rebuilt and the second 2 were new. Each time, I made sure to bench bleed the master cylinder according to instructions (2 methods were used). I also have replaced the wheel cylinders, calipers, shoes, and pads, and it’s still doing the same thing as when I received it. It does have ABS, and I have bled that as well.
O’reilly’s swapped them out without charge each time

If this truck has ABS, it requires a TECH I scan tool to properly bleed the brake system. Without the scanner you’ll never get a firm brake pedal.


Yes, find a qualified mechanic…

Okay, finally what I needed to hear. I shall take it to a brake specialist. Thanks!

Also, next time please let us know up front it already had numerous master cylinders on it in the last two weeks. That would have saved us time and maybe steered you in the right direction earlier.

I did mention that in another reply I made at the same time as yours, but for some reason, it never showed up on the forum. I did, however, fail to mention that it had ABS and I realize that, too, would have made a difference. Anyway, thanks again for your patience and help.

If it really goes ALL the way down, the first thing you should replace is the mechanic.

Now THAT’S FUNNY!!! (dead customers can’t pay their bills) rofl
It’s nice to see a little humor once in a while. Kind of makes my boreing work day here a little brighter. It’s also enjoyable to see smart people on “both sides” of the questions and answers.