Brake master cylinder ….

[YellowBay69]

New thread - Whilst bleeding brakes I can’t get much fluid through driver side front brake and no pressure on pedal. Using the easibleed kit and there are no leaks and fluid passing through all other brakes … so thinking it may some kind of valve return /open situation at the master cylinder. I rebuilt it all a while back with new seals etc.

Have taken it off and have brake fluid squirting through all 3 outlets. Have checked the spring valves on outlet and they seem fine.

Any one have any thoughts on bench testing the pressure outlet on master cylinder or any other tests on the slave cylinders ??

Just trying process of elimination…. any thoughts or suggestions.. would be greatly appreciated.

 

[70-CA-Panel]

Might be a stupid question but did you start bleeding from the furthest away Slave cylinder and working your way closer to the master?
All slaves closed off?

 

[ozziedog]

Try all the different components for free passage, that’s including those pipes, solid and flexis. You should be able to blow through them when disconnected. If you have your original shoes, check the wiggly wiggle pipe between the two slaves. If it’s squirting in your hand on the bench I’d be looking for a blockage or a disturbed slave cylinder rubber.

Ozziedog,,,,,,,,,,,,,, do you really need brakes

 

[YellowBay69]

Might be a stupid question but did you start bleeding from the furthest away Slave cylinder and working your way closer to the master?

Yes Will have another look tomorrow. Try blowing through the line. See if the slave cylinders lock up the wheel ?

 

[ozziedog]

I doubt you’d get the slave cylinders to lock up with just blowing or even an airline bit I’ve Just had a brainwave bud. A little while back, I’d started to try and bleed my brakes backwards because of the fragility of the remote pipe connections, now sorted. I used my small garden pressure sprayer and fidgeted the spray part out of it and fitted a piece of plastic pipe on to it that I could then pressurise the system from the bleed nipple and force air out of it backwards and by myself.
This might prove a point if you can’t get that wheels braking arrangement to accept fresh brake fluid from the other side. I would however try a little just to see if you can get a rise in fluid level in your master cylinder remote pot. If it won’t, then you know there’s a blockage, so don’t pump too much in.

Ozziedog,,,,,,,,,,brakes are for the faint hearted

 

[YellowBay69]

I doubt you’d get the slave cylinders to lock up with just blowing or even an airline bit I’ve Just had a brainwave bud. A little while back, I’d started to try and bleed my brakes backwards because of the fragility of the remote pipe connections, now sorted. I used my small garden pressure sprayer and fidgeted the spray part out of it and fitted a piece of plastic pipe on to it that I could then pressurise the system from the bleed nipple and force air out of it backwards and by myself.
View attachment 8497This might prove a point if you can’t get that wheels braking arrangement to accept fresh brake fluid from the other side. I would however try a little just to see if you can get a rise in fluid level in your master cylinder remote pot. If it won’t, then you know there’s a blockage, so don’t pump too much in.

Ozziedog,,,,,,,,,,brakes are for the faint hearted

Sounds like a plan… will try that tomorrow. Stripped and rebuilt master and cleaned residual valve thingy majigs. Could blow lines backwards while master is off.

 

[YellowBay69]

 

[YellowBay69]

So … refitted the master cylinder. Adjusted up all brakes so pads are touching drum but wheel can be spun then bled all brakes under pressure as Bentley.
What I have is ….
Good oil flow through all slave cylinders.
No leeks.
Can pump up pedal to get resistance at about half pedal stroke.
No further depression or soft if continued pressure on pedal at that point. (no air)
Once pedal released all pressure goes unless pumped up again (pedal almost to floor on single push)
Re read Bentley trouble shoot guide and all things point to master cylinder.

 

[Moseley]

It sounds like a passing seal in the master cylinder. The seals are effectively like an umbrella with the brake side being underneath the brolly, and your foot being on top of it. They should 'open up' against the walls of the master cylinder as the pressure of the brake fluid acts on them. When the seals start to fail, they don't open up fully, and fluid starts to pass - the action of which stops them from sealing and the pedal just sinks to the floor. When you pump the pedal, the rapid movement can be enough to force it to seal, and it will hold pressure.

It's a useful tip if your MC fails whilst driving, but that does sound similar to your symptoms.

FWIW, air in the system can cause it to behave similarly - if you pump the pedal with air present, it will pressurise to the point where the pedal feels hard, so I would be sure that you're happy that you've ruled this out with thorough bleeding.

 

[YellowBay69]

Yes pretty sure there’s no air in system. Just looking at appropriate replacement master cylinder- may get new slaves at same time ….. then EVERYTHING is new !!!

Thanks for your input- really helps getting a second opinion. Especially when i’m strictly amateur ‘driveway mechanic’ !!