No rear brakes

[gordon]

I had my brakes tested at the local garage rolling machine, front brakes fine but no rear at the drums. Tried bleeding the wheel cylinders but no fluid coming out so I assumed master cylinder piston must be sticking on one circuit.
I removed the master and by pumping the piston there is action at all three ports but what concerns me is this. When the MC is mounted the rod from the servo ( which projects 12 mm out from the face of the servo) pushes the mc piston in 12 mm. I'm wondering if the restricted travel of the piton is not allowing the mc chambers to replenish.

Rear brakes have worked ( and leaked fluid) before! So just checking that the servo rod projection of 12 mm is correct

[gordon]

I might be answering my own question here, looking at the workshop manual it looks like the servo rod should be level with the front t casing. So when I fitted the pedal box it may have a too long  pedal operating rod !  ( It was auto originally)

I know some of our members refurbish sevos/ boosters  so it would be great to receive their wisdom.

Thanks in anticipation Gordon W

[badali]

How old are the brake hoses?  If they are very old they could be collapsed and stopping the fluid.  That is the easiest thing to look at first.

[Ed Riefstahl]

Hi Gordon,
I agree with Brad. Either rear hoses or wheel cylinders. Doubtful it's master cylinder.

If you're getting fluid from the rear bleeders, I'd say good chance cylinders are frozen. I just replaced them on my 66 a couple months ago. I'd pull back the rubber boot, mine were full of rusty brown fluid.

Good luck,

Ed Riefstahl
Erie, PA

1966 230SL (Ms Magoo)
1970 280S (Miss Daisy)
1989 300SE (Majestic)
1999 BMW Z3 5 speed
1991 BMW 318I 5 Speed
1997 Toyota Paseo Convertible - Red 5 speed (have you ever seen one?)
1997 Ford Ranger step side (Mater)
2023 Mazda CX 5

[ja17]

Start with new rubber brake hoses.

[gordon]

Thanks for responses. Hoses are braided stainless about 2 years old, not twisted or crimped. No fluid comes out of rear wheel cylinder nipples for bleeding hence my thought that it must be master cylinder.

My question remains, should the master cylinder piston be pushed about 12 mm into the bore when mounted onto the booster/servo. This occurs because the the servo rod sticks out of the servo by about 12mm. So when the servo pushes the mc piston down the bore it cannot return all the way back to the stop because of the projecting servo rod.
Has anyone got a dismounted booster to look at?
Thanks Gordon

[Lorsar]

Is it just the rear cylinders that won't bleed or front as well?.  If just rear cylinders could be this?

https://www.sl113.org/forums/index.php?topic=11971.msg80568#msg80568

[BobH]

I think the link that Lori posted, reply #7 looks to be the most likely cause, the brake force proportioning or regulating valve sticking, here's a picture of it to help you find it

https://www.sl113.org/wiki/Brake/BrakeForceRegulator

[dirkbalter]

I think the link that Lori posted, reply #7 looks to be the most likely cause, the brake force proportioning or regulating valve sticking, here's a picture of it to help you find it

https://www.sl113.org/wiki/Brake/BrakeForceRegulator

His car being a 230 / drum brakes, he does not have the proportional valve.

I looked at pictures of my brake booster and yes, the booster rod is about flush with the front face.

[BobH]

Not wishing to hijack Gordon's original question, but why would a system using rear drum brakes not require a proportional valve?

Thanks

[dirkbalter]

Bob,
from what I understand, the rear disk brakes (250/280) require less brake pressure to operate properly. That pressure differential is achieved through the proportional valve.
I don't really know why the drums don't need it, but assume the nature of the system requires the "unreduced" pressure.
Others may know more about the specifics.

[BobH]

Hello all, if you google, these valves are used on cars with disc brakes all round, or discs and drums, so hopefully someone will know why they were not used on our cars with rear drums

"Proportioning valves are frequently used in cars to reduce the brake fluid pressure to the rear brakes. In cars with disc brakes on the front and drum brakes on the back, more pressure is needed for the disc brakes in the front. The proportioning valve prevents the rear brakes from engaging before the front brakes during panic stops"

I couldn't find anything in the tech manual stating that these valves were not used on 230's, so i've added a note for future reference

[stickandrudderman]

Brake pressure differential valves were not fitted to 230s. I'm sure the Mercedes engineers of the day had good reason not to fit one.

[gordon]

Thankyou Dirkbalter for your picture of your booster showing the booster rod about flush with the front. Am I correct in thinking that the mc piston needs to return all the way to the end stop for the master cylinder to work correctly?

(Because my booster rod projects about 12 mm this would restrict mc piston travel)

[rwmastel]

Sounds like someone needs to 3D print a see-through master cylinder so you can determine where the plunger is going and how it's (not) working. Well, I guess you could measure these things with a MC in hand?

[dirkbalter]

Thankyou Dirkbalter for your picture of your booster showing the booster rod about flush with the front. Am I correct in thinking that the mc piston needs to return all the way to the end stop for the master cylinder to work correctly?

(Because my booster rod projects about 12 mm this would restrict mc piston travel)

12mm sounds like a lot compared to what mine is showing. I believe Stickandrudder who is close to you an commented above, rebuilds these in the UK and can probably help you with a fix?

[Charles 230SL]

..my question remains, should the master cylinder piston be pushed about 12 mm into the bore when mounted onto the booster/servo....

Hello Gordon. No, the master cylinder plunger should not be pushed in by the power brake booster/servo pin when the brakes are not applied. In current state, your booster pin is protruding too far and appears it's keeping the master cylinder (primary piston) replenishing port blocked. If I recall correctly, the primary piston on these master cylinders is for the rear brakes.
I agree with Dirk;12mm is a lot for the booster pin to protrude beyond the casing...I took a look at an old ATE51 booster I have (photos attached). The booster pin is flush with the casing. I'd recommend the power brake booster be rebuilt.

[stickandrudderman]

If a protruding push rod was the problem then your brakes would have been applied all the time.

[gordon]

Thanks Charles for the pictures. I removed my sevo today and the servo rod no longer stuck out, it seems that the arm which is pushed by the brake pedal was unable to return far enough towards the brake pedal. I've removed the obstruction, refitted the servo ( without a projecting pin) and refitted the master cylinder which is no longer partly depressed. I'll change the slightly leaking rear wheel cylinders tomorrow and hopefully be able to pressure bleed the system.

I guess the non fully returning mc piston was not able to replenish the rear brake lines hence no fluid when I was trying to bleed the rears.

Many thanks for all input, where would we be without such helpful members.

[Charles 230SL]

I removed my servo today and the servo rod no longer stuck out, it seems that the arm which is pushed by the brake pedal was unable to return far enough towards the brake pedal.

Oh, I thought the servo was already out of the car - Glad to hear it wasn’t the problem ($)!

[stickandrudderman]

I removed my sevo today and the servo rod no longer stuck out, it seems that the arm which is pushed by the brake pedal was unable to return far enough towards the brake pedal.

In that case you should check for a seized intermediate lever between the pedal push rod and the servo, housed inside the aluminium servo mounting.

[gordon]

Thanks for the advice Stick, the intermediate lever was grazing the pedal box preventing full return. All sorted now and fully bled with new rear wheel cylinders. New MOT certificate now with no advisories.