Update on brake Bleeding.....Purpose of this on our 2800cs master cylinder to the rear brake pipe??

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jjs2800cs

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We have been chasing trying to bleed our fresh 2800cs brake system,
Rebuilt MC (original housing cleaned and was honed), all new pipes, rebuilt front calipers, rebuilt brake proportion valve, new drum brake cylinders.

Took forever and 2 cans of fluid to at least get the front brakes to work. We did pressure, we did vacuum, we did gravity, we prayed to the brake bleeding Gods, but we could not get any fluid to get to the rear. Yes we have read all the past horror stories on bleeding these systems.

So off came the MC and apart again, but all looks ok. Yes we did try to bleed the brake proportion value, but it is not seeing and fluid. We even disconnected the rear brake pipe and connected a short pipe stuck directly into a jar of fluid. Pumping the brakes we got nothing!

So with the MC on the bench again we took a closer look at this extra fitting between the MC and the rear brake pipe, after comparing it to a MC housing from a 1972 3.0CS which does not have one. See photos at the end. Top is 1972 MC bottom is 2800 with valve removed.

Took the fitting off and saw what looks like a ball and spring. Realoem #34314650332 calls it a pre-pressure valve. We cleaned the valve exercised the ball and spring and it seems to allow pressure one way (to the rear pipe) but not the other. Thus it acts like a check valve. Interestingly the 1972 MC housing does not have this fitting so the rear pipe attaches directly.

The valve is no longer available. So we are wondering if ours was stuck and thus did not allow any fluid to get to the rears. So on goes the MC and the fun begins again.

So what is the purpose of this valve? If we end up having the same issue can we just drill out the ball and spring effectively making the MC like the 1972 housing. I suppose we could clean up the 72 housing, hone it and use it.

Why is this even needed? Perhaps it was only fitted to the cars with rear drums?

Thanks

jjs2800cs
PXL_20230708_171818977.jpg

PXL_20230708_171835929.jpg
 
So, back in the 1970s, I wore the brake shoes in my Plymouth down so far that the pistons that pushed the piston rods outward extended beyond the end of the seals on one of my rear brakes. Because I had a single circuit system, this allowed all the brake fluid in my car to exit the car, and I had to limp home with only the ebrake.

I suspect this valve is some sort of safety device that prevents all the brake fluid from leaving the car in situations where the rear brake seals fail.
 
Update, I hate brake fluid

Got MC back on, definitely now getting fluid to the rear but not yet getting the shoes to move. Assuming still got air in the line. Will keep at it. So it seems my valve must have been initially stuck.

As far as what the valve does found this thread on a 2002 site

www.bmw2002faq.com

Master Cylinder Question

I pulled this off of the master cylinder. I am rebuilding it, because I was unable to get the brakes to hold pressure, they would build it but not hold it. But in the process of tearing it apart, I found this. It has the rear brake line connected to it.
www.bmw2002faq.com www.bmw2002faq.com

Here are a couple would would be explanations from the thread

"That is a residual pressure valve for the rear brakes. It keeps a little pressure in the lines for the rears only."

"From one of the 2011 postings--why this valve is interposed between the M/C and the rear (only) brakes:
It helps equalize front to rear brake 'timing' The wheel cylinders have to overcome the spring pressure before contacting the shoes, while the calipers simply slide into contact. Leave it in unless you are converting to rear disc or very bad things will happen
So yes, it's necessary; otherwise there will be a noticeable pause (and a very disconcerting sinking feeling in the pedal) before the rear brakes start working when you press the brake pedal. They don't usually go bad; it may be that the internal valve is stuck and some cleaning with spray brake cleaner will get it moving again."

So in my case it must now be working because when I go to crack the rear bleeder valves (with no pedal movement) fluid squirts out; as the explanation says some residue pressure is maintained at the rears?

Fingers crossed we are close to finishing this.

I hate brake fluid!!

jjs2800cs

Another note for 2800 owners. If you ever have to replace your MC (ref the photos) the 2800 MC has to have a bigger thread opening and machined flat to accommodate the valve and copper seal.

Here is a new one for sale. Its only 1000 bucks but it looks like it has the valve.

BMW Brake Master Cylinder (2 Kreis D=2381) - Genuine BMW 34311120478

BMW Brake Master Cylinder (2 Kreis D=2381) - Genuine BMW 34311120478
www.fcpeuro.com
 
So far this week I have spent about 15 hours across the week trying to get all of the air out of my braking system
She’s Right Hand Drive so a bit different to yours .
There is one thing in common though …
I also hate brake fluid !!!
Or more precisely bubbles in brake fluid.
I hope you get yours sorted
Good luck !!
 
Barry.b said:
So far this week I have spent about 15 hours across the week trying to get all of the air out of my braking system
She’s Right Hand Drive so a bit different to yours .
There is one thing in common though …
I also hate brake fluid !!!
Or more precisely bubbles in brake fluid.
I hope you get yours sorted
Good luck !!
Click to expand...
Start a movement , ban brake fluid , go back to cable friction brakes. Good luck to you I have gone way beyond 15 hrs.
 
jjs2800cs said:
residual pressure valve for the rear brakes
Click to expand...
These are common on cars with rear drum brakes. With out such a valve and/or some other mechanical doodad to keep the shoes close to the drums the brake pedal travel will be unacceptably long as the shoes wear. I have an old car with one of these valves, and it’s a reason why I bleed that car’s brakes old school style (assistant pushing the pedal). It might not be safe to crank up a pressure bleeder with enough pressure at the MC to drive fluid past that spring in the valve, because the pressure might be too much for the brake fluid reservoir/lines.

I’ve installed ez-bleeder valves on that particular car so I can do the pedal push technique when working by myself.
 
Update on brake bleeding

After trying vacuum, pressure, gravity, self-bleeding procedures, I finally got some help and we resorted to the old Down/Up procedure and we got the brakes to bleed. Took a couple of iterations.

Apparently we could not get the bleeder screws to seal when just cracked a bit with the hose in the jar while pumping the pedal. We used grease, Teflon tape but it was not working so must have been sucking air back in past the screw. Anyhow with the down/up method done properly it can prevent any air getting back in.

So finally got brakes and also got the clutch to bleed after 2-1/2 quarts of fluid!

Best of luck to Barry.b with his right hand drive situation.

Still hate brake fluid.

jjs2800cs
 
The vacuum pumps always leak air at the bleed screw and I gave up on them. I have had success with the pressure bleeder, I just use it to hold pressure, I don’t put brake fluid in it.
 
Somebody mentioned ez-bleed valves.
I don’t have them , but they look like a brilliant idea.
I wish I heard about them before.

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Think you should adjust your brake shoes first, then have someone help you bleed the brakes through.pushing the fluid through, you might be able to gravity bleed the rears also as an alternative, I’d try that first after adjusting your brakes shoes first …
 
coupe2800cs said:
Think you should adjust your brake shoes first, then have someone help you bleed the brakes through.pushing the fluid through, you might be able to gravity bleed the rears also as an alternative, I’d try that first after adjusting your brakes shoes first …
Click to expand...
Yes we did do that, adjusting the rear shoes as we went thru the process. Great tip because would be easy to pass over after initial install.

I have ordered the speed bleeders and if we need to do another go around we will give them a try.
Jjs2800cs
 
Barry.b said:
ez-bleed valves
Click to expand...
I’ve tried them. I got them for 2 cars to enable a one person pump-the-pedal approach. My luck was 50/50: worked well on my 56 Alfa (bleeder fit snugly in the wheel cylinders). Did not work on my Jag (bleeder fit was too loose).

Hope you have luck with them (jjs2890).
 
jjs2800cs said:
Yes we did do that, adjusting the rear shoes as we went thru the process. Great tip because would be easy to pass over after initial install.

I have ordered the speed bleeders and if we need to do another go around we will give them a try.
Jjs2800cs
Click to expand...
Doubt the speed bleeders will help here, but let us know what happens, nothing like having a shop assistant!
 
this is for RHD cars - but may work for LHD cars if you have problems ( rhd have 2 servos)



I spoke to a old bmw service chap- and he told me the only way is this way –

Get a brake pressure bleeder like this

https://www.amazon.co.uk/Laser-LAS4832-Manual-Brake-Bleeder/dp/B002ZTRUAW



This then pressurizes the reservoir and this is the correct sequence to bleed is as follows.



This is for RHD Cars only but may work with LHD



  • Top servo
  • Passenger top nipple
  • Drivers side top nipple
  • Bottom servo
  • Passenger back calliper
  • Drivers back calliper
  • Passenger front middle inside nipple
  • Passenger front outside middle nipple
  • Drivers side front rear inside middle nipple
  • Drivers side front outside middle nipple


Then the clutch –

This may need repeating a couple of time …

The pressure bleed machine automaticity fills the reservoir as you bleed.

I have done this once and I now have pressure on peddles.

Also, I was told that sometimes with the clutch you may need to pressurize it from below???



I think with the RHD setup this is the only way to pressurize from the top…

But there may be someone out there can tell me differently
 
HB Chris said:
They leaked like the normal ones for me.
Click to expand...
They sounded like a great idea, but I will keep extra beer in frig and whatever other necessary spirits for car buddies who don't mind pushing the pedal a zillion times.
After trying for days by myself, in an hour or so down up worked, so the speed bleeders go into storage.
Jjs2800cs
 
Do you mean you used the speed bleeders and after using them the job was finished in an hour ?
Or do you mean you tried the speed bleeders and as they didn’t work you stopped using them after an hour ?
 
Barry.b said:
Do you mean you used the speed bleeders and after using them the job was finished in an hour ?
Or do you mean you tried the speed bleeders and as they didn’t work you stopped using them after an hour ?
Click to expand...
Did not even try to use the speed bleeders as we received them after ( delayed in shipping to me) our successful bleeding using two people. If in the future I need to bleed the car again, I will give them a try, but based on others saying they had little luck with them, we will not spend a lot of time getting them to work.

jjs2800cs
 
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