Occasional brake problems

[jeremycfr]

We converted my car to front disc brakes. We removed the necessary parts from another car except for a new MC.

We have a good brake pedal, are confident we have no air, and we've tested the MC.

Randomly the pedal will go all the way to the floor. Thoughts?

Also, the brake pedal does not always rerun high enough to disengage the brake lights.

 

[c9zx]

It sounds like an internal leak, also called internal by-pass, in the master cylinder to me. I've seen a few times, mostly with "rebuilt" master cylinders.

Chuck

 

[turtle5353]

It sounds like an internal leak, also called internal by-pass, in the master cylinder to me. I've seen a few times, mostly with "rebuilt" master cylinders.

Chuck

YEP! +1

 

[Manu Mach1]

It sounds like an internal leak, also called internal by-pass, in the master cylinder to me. I've seen a few times, mostly with "rebuilt" master cylinders.

Chuck

Means that fluid from MC could move from one compartment to the other?

Or what?

 

[c9zx]

Yes. This explains it reasonably well. http://books.google.com/books?id=H922It35HOMC&pg=PA92&lpg=PA92&dq=master+cylinder+internal+leak+bypasing&source=bl&ots=bUXyPVXxgG&sig=ia1u9B0M99n-vcHn7hx7-Krcz4w&hl=en&sa=X&ei=yL2lU5LoNYOuyATO5oGYAg&ved=0CB8Q6AEwAA#v=onepage&q=master%20cylinder%20internal%20leak%20bypasing&f=false

Chuck

 

[Manu Mach1]

And again I would say, every day we learn.

 

[jeremycfr]

We replaced the MC, bled the system again,have good pressure at each tire, and cannot find a leak. Any other thoughts?

Maybe wrong length on the pushrods?

 

[mustang7173]

I think c9zx has a valid solution to your issue. I have read also that the bore size has an effect on the pressure of the brake system. I would think that replacing the master cylinder with a new one would solve the issue. Check out the Master Power Brakes website here!

Master Power Brakes > P/B Master Cylinder FAQ's

http://www.mpbrakes.com/master-cylinders-faq

mustang7173

 

[c9zx]

We replaced the MC, bled the system again,have good pressure at each tire, and cannot find a leak. Any other thoughts?

Maybe wrong length on the pushrods?

Typically if the rod is too short you will have excessive pedal travel and if it is too long the brakes will drag. You should check it buy building a sheet metal gauge "block" as described in the Ford manual, chassis volume, 12-01-04. It is a precise adjustment. Chuck

 

[OMS]

We replaced the MC, bled the system again,have good pressure at each tire, and cannot find a leak. Any other thoughts?

Maybe wrong length on the pushrods?

What did you do at the firewall?

The drum MC bolts on and I think has a spacer to hold the threaded blocks.

The disc booster has studs - no spacer but has a sheet metal plate.

It was years ago but I remember having to disassemble a converted car to swap plates as the pedal travel was off, brakes were very touchy.

 

[jeremycfr]

We replaced the MC, bled the system again,have good pressure at each tire, and cannot find a leak. Any other thoughts?

Maybe wrong length on the pushrods?

What did you do at the firewall?

The drum MC bolts on and I think has a spacer to hold the threaded blocks.

The disc booster has studs - no spacer but has a sheet metal plate.

It was years ago but I remember having to disassemble a converted car to swap plates as the pedal travel was off, brakes were very touchy.

Don

We replaced all under dash parts with those from my parts car. We then took the booster and bolted to the firewall, replaced the control valve, and bolted the new (not rebuilt) MC to it. Both MCs I now have bench tested ok today.

 

[jeremycfr]

We think we've figured out the problem. Yesterday we re sealed the gear box, replaced the pitman arm, replaced the idler arm, and aligned the front end (I'm gonna miss having access to a full shop).

While we were up there we looked to see if the new disc brakes were working and at all times everything was fine. That told us the problem was in the rear.

We pulled a vacuum on the rear brakes, but no matter how much I pushed the rear brakes never engaged. Sine the MC tested ok, we think the problem lies with the proportioning valve not allowing enough pressure. We've ordered a new one and hoop fully it will solve the problem.

The exciting part was watching the car move off and on the rack by itself for the first time in 20+ years.

 

[jeremycfr]

Fixed!! It was the proportioning valve. Whoo Hoo I may get to drive her this week.