Bleeding brakes

[MT]

... in the process thereof. 1966 SWB single MC, new seals/internals, new pipework etc. DOT 4 fluid

Had a lot of trouble getting the fluid to flow through the cylinder at first, but after a dismantle, check and rebuild all seemed to be going well, and now have a pedal after a few pumps. Problem is the system holds the pressure and consequently the brakes lock on. If I open any of the bleed valves fluid flows under pressure into my bleed container and the brakes become free - not too surprisingly! Facilitates getting the air out at least ......

Seems to me that the new 'special check valve' that is on the innermost end of the MC spring is not functioning - necessitating a drain, dismantle, clean, fix, rebuild, rebleed etc. etc. I have left it under pressure to see if it pops free, but even if it does I think prudence suggests a dismantle anyway. The thin washer is present between piston and primary cap, and the piston is the right way round.

Any other possibilities before I get that bleeding awful bleeding fluid all over the bleeding car, me and the bleeding garage floor??

Mick

[Lightweight_911]

Is it the same situation at all 4 corners ?

[MT]

Well certainly both fronts - I must admit I never checked the rears as they are still on the ground on their wheels so not easy to check independently.
I just assumed that as the RH front is pressurised by the same piping as both rears it was likely to apply to all.

Mick

[911hillclimber]

Check you have enought free pedal play befor ethe plunder moves the cylinder piston.

I had mine too tight and the pads locked on when I changed my master for a new one..

Giving more play fixed it.

73T: Acalipers front/M rear stock everywhere else.

[MT]

Thanks.

Ed Mayo just raised a similar point on 911S where I also posted my question (a bit more politely I confess). I didn't check the free play, thinking I would do as part of a final adjustment (along with e-brake etc etc), but it seems it might be critical to this, although I confess I cannot see how the physics/hydraulics work to cause this. Anyone willing to explain would be appreciated. I like to know how these things work - helps in future problems I find ...

Presumably if I just detach the plunger from the pedal and manually pump the cylinder with a long screwdriver or such that should check if this is the cause?

Mick

[smallspeed]

as i understand it..

in very basic terms a master cyliner has two ports, one connecting to a reservoir and the other to the piston (or pistons) in a caliper(s). when the pedal is in the rest position, fluid from the reservoir is connected to the master cylinder and the caliper piston, and the system is at atmospheric pressure. when you you push the pedal, the piston in the m/c moves down the cylinder blocking the port connecting the cylinder to the reservoir. once this port is fully closed, pressure is built up between the master cylinder and the brake caliper, forcing the piston out on the caliper and applying the brakes (the reservoir remains at atmospheric pressure).

as you take your foot off the brake, the return spring pulls the mc piston back-up the cylinder withdrawing the brake caliper piston, until the port to the reservoir is open again, at which point the whole system is open to atmospheric pressure again, allowing the reservoir to "back-fill" any lost fluid

if you get the position of the piston wrong at the rest state, you can either lock the brakes on (there's no release to atmosphere) or cause excessive pedal travel due to the pedal needing to cover the ground between rest and closing the port.

* ideal adjustment is usually so the piston is level with the start of the port to the reservoir, or partially covering the port (usually less than 1/3 closed to allow decent and fast pressure equalisation)

hth

ben

* this is based on what we try to achieve at work, not what you should do on your 911, but im sure someone will have a decent pedal / mc adjustment procedure for you

[911hillclimber]

Above is sound to me.
If you adjust the plunger so there is a good distance on the pedal, say 20mm then you will bleed it all out.
You need to be careful not to try to get modern car pedal play as it will not work.
That is the trap I fell into last time about 8 years back when I changed the master.

When done, check the brake light switch point!

[MT]

Good explanations, thanks.

Adjusted pushrod until it is 5mm short to be sure - still holds pressure.
Drained, dismantled, checked the m/c internals - changed to slightly different piston that I had from a spare 911 m/c - same length but slightly different shape - still holds pressure
Only thing I haven't swapped is the 'special check valve' at the innermost end of the spring. Any other thoughts before I drain/dismantle again?

I hate brake fluid.....

Mick

[hot66]

you deffo not got any restrictions in the lines / flexi's ?

[MT]

No checked them, and the pressure is held on all four callipers - when I open the bleed valve it doesn't matter which corner I use - which to me points to the m/c.

Mick

[MT]

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 in length.
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

[MT]

Problem sorted - I think

Anyway I now know why the main bore didn't fill. For fluid to pass from the piston area to the main bore there needs to be a slight vacuum in the main bore. This is easily created when the system is tight as you release the pedal, but if there's a leak into the system it prevents the vacuum, and no fluid goes into the bore. I found this out by bench testing. If you put just enough fluid to fill the tiny cup for the reservoir plug, and pump the piston the fluid stays put, but if you seal the two outlets with your fingers on the 'out' stroke the fluid is sucked quickly into the cylinder. Do this a few times and fluid then exits the outlets. Embarrassingly I found that the RH calliper bleed valve wasn't tight (from my last round of bleeding I suppose), hence no fluid from reservoir to main bore.

Anyway with that closed and with the small hole in the 'special check valve' confirmed clear the system operated normally - no brake lock on when pedal released.

I can only assume that the hole in the check valve was blocked and that was the cause of the problem.

Now I have air locks everywhere from so much dismantling, so more fluid required, but I have the beginnings of a pedal so I am hopeful that the problem is fixed. If anyone wants a litre of used DOT4 I'm open to offers....

Thanks to all

Mick

[MT]

Perhaps premature ejac.., I mean congratulation on my behalf. As I get more of the air out of the system, it is starting to hold pressure again. Not enough to lock the wheels yet, but enough to bind the brakes a little. It is clear when I first open any bleed valve that there is some pressure in the system as fluid flows out along the bleed tube before the pedal is depressed (like me!).

Have decided to leave it for the day and have a fresh start tomorrow so any thoughts welcome.

Mick

[911hillclimber]

I struggled to do my brakes a few years back after a total calliper reseal/pipes/discs etc.
I bought a new MC from Autofarm and had the plunger length issue previously described. It took ages bleeding them out after the pedal play wes sorted.
Got them close but still 'pumpable' and left them over night to return to a rock hard pedal the following day.

I doubt the master needed changing, but it was done.

So, I have a good spare 18mm dia master you are welcome to borrow to see if that simply works.
It has never been apart from new, it is about 60k miles old.

If you want to try it and the car works with it you can send a donation to the excellent !0 years of DDK Charity Fund if you want to leave it in.

Let me know with an address and I'll post it to you if you wish, no probs if you would rather not.

Graham.

[Mike911scrs]

HI MT
what flexi 's have you on the car??? new, old rubber type or steel braided, if Std rubber type how old are they.

regards mike