Single m/c special check valve

[MTemp]

Single M/C system, 1966 swb, new pipes etc.

In the process of bleeding the brakes and at first I had difficulty getting fluid to go a) into the m/c and then b) long the brake lines. After a removal, check, refit I eventually got fluid to flow, but now I find the system holds pressure once I stop pumping the pedal, and so the brakes lock on. Opening any bleed valve on the callipers into my bleed container releases the pressure and the brakes free up (not surprisingly!)

Am I right in thinking it's the 'special check valve' (as described in the Haynes manual item 2 Fig 8.19) that is probably malfunctioning? I tried to clear the small 'purging passage' before fitting as described in the same manual, but it's so small it's difficult to be sure it's clear.

It's just I'd like to know all the possibilities before I drain, dismantle, etc etc and get that awful stuff (DOT4) everywhere...

Thanks

Mick

[edmayo]

Did you adjust the master cylinder pushrod to have one millimeter of clearance when the pedal is all the back?

[MTemp]

No I didn't Ed, good point.
It's the same m/c (new internals) and rod as original, but I'm sure the measurements will have changed, although not by much I'd have thought.

Not sure I can see how that would cause this result. If you can be bothered please explain....

Thanks for the quick response

Mick

[304065]

Ach, Ed has beaten me to the answer! (Texans wake up early!)

Take a look in the factory workshop manual at the sideways cross section of the master cylinder. This is the single circuit so it's very easy.

The piston slides to the rear (of the car) and uncovers the port to pick up fluid from the reservoir. Then you push the brake pedal, the port is closed off and pressure builds in the brake circuit. You release pressure and the piston slides back.

This is why 1mm clearance between the actuating rod hooked to the pedal lever and the face of the piston is critical-- there must be room for the piston to slide toward the rear of the car under all operating conditions, cold or hot. If it doesn't, and the actuating rod holds it forward, then it will continue to hold pressure resulting in a lot of brake dust and reduced lap times!

[edmayo]

Ahhhh! but your explanations are so well worded! I'll do the one-liners then let you eloquently expound

[MTemp]

Sorry Ed/John,

Great explanations I agree, but not the solution. I have adjusted the pushrod, no change and drained/dismantled/checked the m/c again - and still the system holds pressure after the pedal is released - certainly both front wheels, and given the system layout therefore probably all 4 (the rears are on the ground).

The only thing left to swap is this 'special check valve' on the innermost end of the spring.

Any other ideas before I drain/dismantle again?

Mick

[edmayo]

Was this a new master cylinder from ATE or one that was 'rebuilt' ?

[MTemp]

Rebuilt original Schafer. Bore was perfect (it clearly produces and holds pressure!).
Rebuilt with a kit from a supplier in Belgium, although by now I have swapped quite a few components with another spare m/c I had.

Mick

[MTemp]

Picture - thousand words etc etc

This is a photo I took about a year ago when I dismantled the m/c - contents in order


And below is a photo of the various bits I have tried in the rebuild.



Currently the bottom row is installed. Spring is 80mm long.
Current situation (after making sure the pushrod isn't the problem) and ensuring the little hole in the 'special check valve' on the far left above is clear is that fluid will not flow from the reservoir into the cylinder (this is what happened on my first attempt). I have checked all holes are clear, pipe from reservoir to cylinder is clear and untwisted, and when I dismantle the void around the piston is filled with fluid, but the main chamber is dry, so fluid is not getting past the primary piston cup. How is it supposed to flow with the small holes in the piston blocked by the thin washer? Is this washer supposed to be curved in some way, because the new one is dead flat?

So I can't even get back to having locked brakes....... suspect the two conditions have the same cause ....

Puzzled

Mick

[uai]

pipe from reservoir to cylinder is clear and untwisted,

Is there at the entry to the mc everything as it should be?
Do you have the little washer beneath the reservoir plug?

Looked it up. It's #9