W123 occasional soft brakes

[sceh]

1981 240TD auto with 75K miles.
The master cylinder is new-ish and the front calipers and pads are brand new and the rears have been overhauled.
Engine off, press the pedal and it is hard. Start and it descends slightly.
Sometimes when driving, the pedal movement will go from hard to very soft with a lot of travel. Pumping restores it a bit. If I press the pedal hard at a standstill and then move off and brake, it brakes well. Then once it has braked once the pedal will be hard but the braking very poor.
With this information, where would you start to diagnose things?
It doesn't sound like it needs bleeding since it was recently done anyway. There are no visible leaks. The sudden softness and brake travel is unnerving to say the least.
Brake power assist unit? Master cylinder?
Thanks for any pointers. I found out about this on the way to an MOT test after doing some emergency braking to test things out on the way..

 

[Bellow]

If bleeding was required I'd suspect that the pedal couldn't be better than mushy at all times - never hard. I'd start by investigating the servo side of things looking for leaks that allow the vacuum to deplete (I'm assuming there is a vacuum reservoir - probably looks like a plastic egg box) and the pump (assuming again there is one but maybe not as I suspect your car has a throttle valve).

 

[johnsco]

If it was mine, I'd bleed it thoroughly anyway.
I've often had brakes that didn't feel quite right, and bleeding has made a big improvement.
Good thing is ..... It doesn't cost much and it can only improve things and/or eliminate one possible problem.

 

[grober]

Master cylinder/ Brake Booster seals at a guess
Mercedes-Benz W123 Brake Master Cylinder Replacement | W123 (1977-1985) | Pelican Parts DIY Maintenance Article
Untitled Document

 

[sceh]

Thanks for all the constructive replies.
I suspect the vacuum somehow since I have been messing around with the tubes since the transmission flared a lot - caused by the local Bosch garage swapping a couple of lines to the switchover valve btw!
At the rear of the car there is a very large eggbox which stores vacuum. The pressure at eh servo is 22-23 inches of vacuum so the pump is OK.
Any hints from here on? I will block of the T-junction on the way to the servo so the pump only sypplies the servo and see what happens. I am not sure how to test the eggbox and am not even sure how it is fed. Any hints here?

thanks

 

[sceh]

Since it is normal that the pedal would be hard if the servo was duff, what explains the pedal going to the floor and not stopping the car occasionally?

 

[grober]

CHECK THOSE RUBBER INTERCONNETS!

FROM
W123 Vacuum InterLock Diagram - PeachParts Mercedes-Benz Forum

 

[grober]

PEDAL TO THE FLOOR IS USUALLY A DODGY MASTER CYLINDER. Don't think they are too expensive- I would replace as a matter of course together with any associated brake booster /reservoir seals .

 

[Bellow]

My petrol W123 had the egg box for central locking but the diesel it seems uses the same one for both locking and brakes. Not sure if mine served the brakes as well as the locking but the door actuators leaked the vacuum away very quickly - such that I had to employ 'technique' to get it lock all doors at once.
The rubber seals in the door actuators perish and obviously any other rubber connectors can also. There could be a pipe trapped/kinked interfering with operation. I'd focus on the vacuum side of things as per the things Grober highlighted.

 

[Bellow]

There appears to be possibly two separate problems. One is the 'hard' pedal - vacuum/servo related. The other.....

PEDAL TO THE FLOOR IS USUALLY A DODGY MASTER CYLINDER. Don't think they are too expensive- I would replace as a matter of course together with any associated brake booster /reservoir seals .

.....as per above.
Worth mentioning perhaps that bleeding can rip up a master cylinder seal if there is corrosion at the far end of the cylinder bore not normally swept in use but during bleeding when full pedal travel is used, the seal encounters the roughness of the corrosion. Happened to a friend's Granada after some brake work. Pedal to the floor in his case.

 

[sceh]

One step forward and two back...

I have found no leaks anywhere - good news I suppose..
I disconnected the vacuum to the servo by removing the small pipe from the T-junction in the feed from the pump to the servo. The braking effect with or without is not noticeably different - perhaps a bit better with the pipe connected but nothing radical. The pedal is still hard and the brakes lack bite - curiously there is more bite in reverse in spite of the pads, calipers and discs being new at the front.
When I start the engine with the pedal pressed hard, the pedal goes down a bit and if I keep the pressure on it will descend a bit more very slowly.
I would have expected the brakes to be much better with the servo than without though. I am assuming the disconnection I did is enough to 'kill' the servo?
None of this points to why I had the sudden softness unless this could be due to some internal glitch in the MC causing fluid to flow past the seals and not compress?
I need the car to stop on a sixpence (asking a bit much of a 240D perhaps? )
Short of changing the MC and servo, does this info give you any more insights?

thanks

 

[sceh]

Perhaps a dumb question but here goes.
The current master cylinder is Bosch but the fluid container is ATE. The servo is Mercedes original.
What braking system do I have? ATE or Bosch? I told you it was a stupid question!

If it helps, the VIN is WDB12318312013266

 

[grober]

The anomalous brake behaviour is probably to do with the dual circuit nature of the brake master cylinder primary and secondary circuit seals. GET A NEW BRAKE MASTER CYLINDER------ don't even consider rebuilding the old one.

 

[Bellow]

The once I had a master cylinder go bad on me was on an unservoed (1275 GT Mini) car and the symptom was the car not stopping as expected in the final stages (as the pressure bled away). As mentioned before, failure can be worse than that.

My current car is nigh on unstoppable without servo assistance but it clearly varies from car to car. The pedal moving underfoot on start up happens on my car too. Bear in mind, even with servo disabled, it stores vacuum for a couple of stops - then it's done.

Servos apparently have a rubber cushion sort of thing inside through which pedal effort is transferred. To my understanding it is the deformation of this that invokes the servo. Maybe yours is perished or somehow not as it should be.

The master cylinder though seems to be the primary suspect here but I would have another look at the pad installation. Disc brakes are self-servoing to a very small extent (tapers off with pad wear). That you are getting better braking reversing may be a clue.

'Stopping on a sixpence' should be the reality. My W123 200 had excellent brakes.

 

[grober]

Perhaps a dumb question but here goes.
The current master cylinder is Bosch but the fluid container is ATE. The servo is Mercedes original.
What braking system do I have? ATE or Bosch? I told you it was a stupid question!

If it helps, the VIN is WDB12318312013266

IT DOESN'T MATTER as long as you get the correct part from either manufacturer it should fit [ I would favour ATE] - remember to renew the two seals between the dual reservoir and master cylinder and the o ring between the master cylinder and servo A new master cylinder probably wont come with these as standard.
ps because brakes are a primary safety issue I would entrust the repair to the pros. Any decent general/all makes service garage or merc specialist should be able to do this for you

 

[alabbasi]

If it's the booster, it will likely suddenly go hard. It's either a bad master cylinder or your hoses are starting to creak down and expand as you apply pressure.

 

[sceh]

Bought a new master cylinder (ATE) and fitted it but am having trouble bleeding it since there is no bleed valve, just the three connections and of course I didn't bench bleed it before fitting it...
Any hints please?
thanks

 

[grober]

Mercedes-Benz W123 Bleeding Brakes | W123 (1977-1985) | Pelican Parts DIY Maintenance Article

 

[Bellow]

I'd have thought the master cylinder self bleeds by displacing air (with fluid) and venting back into the reservoir. I'd open the nearest wheel nipple to encourage flow and then the others in turn to 'gravity' bleed to begin with. Thereafter, choose your method as per Grober's linked piece.
I don't recall ever having to do anything different than the above to bleed a depleted master cylinder.

 

[sceh]

Changed the MC and bled the system the old fashioned way and the brakes are now pretty good. The front calipers, discs and pads are brand new as well so now to the testing centre to see if it passes! Thanks for all the advice!