Brake pedal to the floor

[GaryOR]

1978 Scout II rebuild.
Replaced master cylinder. New front disc pads and calipers. New front and rear center brake lines.
All lines cleared of air. Emergency brake works.

However, when bleeding lines, brake pedal goes all the way to floor. When lines all closed, brake pedal still goes all the way to floor. As a result, no brakes at all.

I don't know anything about the brake booster, so don't know if that is somehow a problem.

Suggestions?

 

[Hondo]

No pedal resistance at all? With all you replaced, make sure and check for leakage everywhere, doesn't take much. If you have a leak you, are sucking air back in every time the pedal comes up, and you will never get any pressure. Need to have someone step on the pedal hard while you are looking for leaks. Also I'd look in the forum for info on bleeding the new master cylinder.

 

[Scooter]

Did you bench bleed the new master cylinder before you installed it? It can be done while it's mounted in the vehicle using the brake pedal, but before the hard lines are installed.

As mentioned, there could still be an air leak down line from the master. Or the system isn't bled well enough

 

[GaryOR]

So from these responses it seems that it is a bleeding/air issue and not anything to do with the power booster. Correct?

I get some initial midway pedal pressure, then it gives and sinks completely to the floor. I do, however, have enough pressure to shoot fluid out at the ends of the lines.

How do you bleed the master cylinder differently than the whole system?

 

[Scooter]

Only major issue that would lead to the booster is the lack of vacuum. And/or the diaphragm inside the booster itself. One other thing, would be the union connection between the brake pedal and the booster that May have been loosely disconnected.

For doing a bench bleed of the master cylinder, I use a couple of short pieces of hard line with the proper fittings and flare to be screwed into the master. Then route the opened end back up into the reservoirs. For me, it's easier to mount the master cylinder onto the booster and use the brake pedal. Filling up both reservoirs with fluid and pump the brake pedal to cycle the fluid through the master. I thought I had a spare master cylinder laying around, but here's a snap shot of the short pieces of hard line/fittings I use.

 

[GaryOR]

I disconnected the lines from the master, covered the holes, and pumped the brakes many times, getting air bubbles back into the reservoir until no more.

Reconnected lines. Re-bled system. Some improvement, but still can push the pedal to the floor. And no resistance to spinning rear wheels when jacked up.

No visible leaks.

Other suggestions?

 

[FDChappie]

Sounds to me like you've got a bad mc. I'd try another one.

 

[GaryOR]

Thanks for all the help.

Solved. Finally bled the system enough to get all of the air out and fixed small leaks at the front caliper attachment.

Rear brakes fixed by replacing the slave cylinders (only $10 each).

The picture shows what the old ones looked like. Yes, I dumped sand out of them.