Midget Brakes will not hold pressure

[Cujo]

Hello (moved from mistaken post on MG Forum - my bad),

I am ready to run my 1970 Midget off a cliff. I have replaced all wheel cylinders and the master cylinder and rebuilt the front/rear switch. I still cannot get the system to hold pressure (all lines are bled and all cylinders are braking). That is when the pedal is pumped all the brakes work but as soon as you stop pumping for a second the brake pedal then goes straight to the floor with no pressure - and as I said it is a new master cylinder and it has been bench bled and all wheel cylinders are showing clear fluid with no bubbles.
Any ideas or experience is much appreciated.

Thank you

 

[Trevor Jessie]

Sounds like a bad master cylinder, but that is just a WAG.

 

[Sarastro]

Yes, it sounds like that to me, too. Might still be bleeding, but in my experience, the brakes aren't too difficult to bleed. (The clutch is another story.)

I assume that the rear brakes are correctly adjusted?

Dunno about Spridgets, as I have a Bugeye with drum brakes all around--but on some cars (the Porshe 911 and 912 are notorious for this) there's a "hysteresis" effect. The seals in the calipers are designed to retract the pads in the disk brakes slightly, after you take your foot off the brake pedal, to keep them from dragging on the disks. This effect can be overdone; they retract the pads too much, and you get the phenomenon you're describing. The solution is to take out the pads, push the cylinders close to the disk, then move them back just enough to put the pads back in. Probably this isn't your problem, but I thought I should mention it just in case. It's easy to check.

 

[Bayless]

If I understand right you are saying you can pump up pressure but eve3n that will go away if you stop pumping and just hold your foot down. If right then it almost surely is fluid leaking past the seal in the master cylinder.

 

[Gerard]

That is when the pedal is pumped all the brakes work but as soon as you stop pumping for a second the brake pedal then goes straight to the floor with no pressure - and as I said it is a new master cylinder

This is almost certainly a bad or poor quality seal. I have had this happen before on sleeved and rebuilt MC's. Always turns out to be one of the following:

• seal under spec. diameter
• a small piece if debris stuck between the lip seal and the cylinder wall
• improperly installed or torn lip seal.

Who supplied you with the product?

 

[MikeMcKay]

I would be sure rear brakes are adjusted. If nothing else adjust until they lock the rear brakes.
Then try your brake pedal. If if still leaks down you have problems with the master cylinder. If pedal holds it is just adjusting the rear brakes.
Did you change out front calipers? I wasn't sure as you said all wheel cylinders.

Mike

 

[JPSmit]

He hasn't updated this thread but on the MG board a new MC solved the problem