Brake pressure regulator 2800cs

[Mich]

Hi,

I'm try to sort something out. I've an 2800cs from 1970. Last year I upgraded the brakes to 3.0 csi brakes, also the rears. Does anybody know if the brakes pressure regulator (in the rear) from a 2800 is the same as an 3.0?


I look that my rearbrakes have more pressure then the front one. Yes I also changed the brake master cilinder.


Thanks!

 

[Peter Coomaraswamy]

Hi Mich, I'm thinking of doing the same conversion. The regulator looks exactly the same, I would have to check the part numbers. Was there a reason you changed the master cylinder?

Maybe others can add to the discussion!

 

[bavbob]

Closed system with non-compressible fluid and a functional master cylinder. Worst case would be almost equal distribution to all four corners (less the min resistance differences from line length). Could you have dislodged something while changing the MC so a line is blocked? Assume you bled the system.

 

[lloyd]

Not aware that the two devices are different. However, that does not mean they are calibrated the same. Sorry, it's been decades since I thought about such things. It is also possible your device is not functioning properly. The factory manual suggests measuring the pressure, but when I encountered something similar, I swapped the limiter with a spare.

Brake pressure limiter - checking
Connect a high pressure gauge to the front end rear axle brake circuit and a second gauge to the rear brake caliper. Depress the brake pedal and hold down with the pedal prop.

Block outlets from brake fluid reservoir with two pointed wooden sticks. Remove rear left brake hose (34 32 551). Unscrew brake lines from brake pressure limiter. Remove brake pressure limiter.
Never attempt to alter the setting of the brake pressure limiter.

Until a pressure of 20 atü (285 psi) is registered, both pressure gauges must indicate the same reading. From 20 atü (285) upwards, the pressure at the rear fixed caliper should be correspondingly reduced as shown in the diagram. A output pressure B input pressure C tolerance range If the measured values differ from the nominal values, the required rear axle pressure can be set with the adjusting screw when the brakes are released. Screw out - increases rear axle pressure Screw in - reduced rear axle pressure Prevent the adjusting screw and castellated
nut from turning with Klebert M 28 sealer.

http://www.e9-driven.com/Public/Library/BMW-E9-Manual/pages/en/34341000.html

The regulator looks exactly the same, I would have to check the part numbers. Was there a reason you changed the master cylinder?

There are two primary reasons for changing a working master cylinder. The bore of the 4-disc master cylinder is slightly larger than its 2800cs counterpart. A larger bore in the same pedal configuration results in greater brake fluid volume displacement - with the same pedal stroke. This can result in slightly increased pedal effort. This does not mean that the smaller bore master cylinder wouldn't work with the conversion, any more than fitting a different steering gear box with an altered gear ratio wouldn't work to steer the car. Depending upon the use of the vehicle, some operators might not notice the difference. (Of course there are some drivers who might notice a change of .5 lb/sq.in. spare tire pressure!)

A secondary reason for changing the master cylinder when converting from a disc-drum combination to a 4 wheel disc system might be the one-way check valve that fits between the master cylinder and the rear brake lines. The valve maintains a minimal amount of rear line pressure to drum brakes to counteract mechanical spring pressures. Consider that drum brakes typically have mechanical return springs, forcing brake shoes to retract and avoid contacting the drums. Disc brake design precludes the need for the mechanical spring pressure, and, consequently, the 4-wheel disc braking system does not employ a check valve to the rear wheels. Removal of the in line brass-alloyed check valve is advised when converting from drum to disc brakes.

The so-called pre-pressure check valve is "no 1" in the diagram below.

HTH

 

[Mich]

Closed system with non-compressible fluid and a functional master cylinder. Worst case would be almost equal distribution to all four corners (less the min resistance differences from line length). Could you have dislodged something while changing the MC so a line is blocked? Assume you bled the system.

I'm sure nothing is blocked. I changed de MC with no extra reason. The 2800cs will work if you remove the one-way block. And yes, I bled the system.

 

[Nicad]

Not aware that the two devices are different. However, that does not mean they are calibrated the same. Sorry, it's been decades since I thought about such things. It is also possible your device is not functioning properly. The factory manual suggests measuring the pressure, but when I encountered something similar, I swapped the limiter with a spare.

http://www.e9-driven.com/Public/Library/BMW-E9-Manual/pages/en/34341000.html

There are two primary reasons for changing a working master cylinder. The bore of the 4-disc master cylinder is slightly larger than its 2800cs counterpart. A larger bore in the same pedal configuration results in greater brake fluid volume displacement - with the same pedal stroke. This can result in slightly increased pedal effort. This does not mean that the smaller bore master cylinder wouldn't work with the conversion, any more than fitting a different steering gear box with an altered gear ratio wouldn't work to steer the car. Depending upon the use of the vehicle, some operators might not notice the difference. (Of course there are some drivers who might notice a change of .5 lb/sq.in. spare tire pressure!)

A secondary reason for changing the master cylinder when converting from a disc-drum combination to a 4 wheel disc system might be the one-way check valve that fits between the master cylinder and the rear brake lines. The valve maintains a minimal amount of rear line pressure to drum brakes to counteract mechanical spring pressures. Consider that drum brakes typically have mechanical return springs, forcing brake shoes to retract and avoid contacting the drums. Disc brake design precludes the need for the mechanical spring pressure, and, consequently, the 4-wheel disc braking system does not employ a check valve to the rear wheels. Removal of the in line brass-alloyed check valve is advised when converting from drum to disc brakes.

The so-called pre-pressure check valve is "no 1" in the diagram below.

HTH

Good info there. I am going to check to see if my 2800 that has been converted to rear discs has this removed. I am thinking of just using an adjustable pressure regulator I already have.

 

[gwittman]

I don't know if this will help but when I converted from drum brakes to disc on the rear of my Austin Healey Sprite race car, I had to install an adjustable pressure regulator on the brake line going to the rear. It just controls the pressure to the rear as a percent applied to the front. Once I got it adjusted properly by trial and error, I have very good balance front/rear. Without the pressure regulator, the rear would lock up too easily.

I do change the bias slightly to the rear in wet conditions to make sure the fronts do not lock up first.

 

[lloyd]

I don't know if this will help but when I converted from drum brakes to disc on the rear of my Austin Healey Sprite race car, I had to install an adjustable pressure regulator on the brake line going to the rear. It just controls the pressure to the rear as a percent applied to the front. Once I got it adjusted properly by trial and error, I have very good balance front/rear. Without the pressure regulator, the rear would lock up too easily.

I do change the bias slightly to the rear in wet conditions to make sure the fronts do not lock up first.

With tongue-in-cheek, the manual describes adjusting the limiter while severely frowning upon any attempt to do so. Obviously the consequences of maladjustment could be serious. Nevertheless, it does more than hint that adjustment is available. http://www.e9-driven.com/Public/Library/BMW-E9-Manual/pages/en/34341000.html

Block outlets from brake fluid reservoir with two pointed wooden sticks. Remove rear left brake hose (34 32 551). Unscrew brake lines from brake pressure limiter. Remove brake pressure limiter. Never attempt to alter the setting of the brake pressure limiter.

If the measured values differ from the nominal values, the required rear axle pressure can be set with the adjusting screw when the brakes are released. Screw out - increases rear axle pressure Screw in - reduced rear axle pressure Prevent the adjusting screw and castellated
nut from turning with Klebert M 28 sealer.

The science behind brake proportioning is deceptively simple, but there is far more to it. http://www.stoptech.com/technical-support/technical-white-papers/proportioning-valves Except for a very specialized situation, e.g., where it might be necessary to balance each individual brake, as opposed to two fronts and two rears, an additional pressure regulator needlessly complicates the system and adds the likelihood of unexpected complications. However, a strategically placed aftermarket pressure regulator (as a replacement for a defective-poorly adjusted original) could make things much easier to set up - especially at a track.

http://www.wilwood.com/MasterCylinders/MasterCylinderValves.aspx

Z28 installation.