I am driving a 1980 240d that has a 1977 240d engine transplant.

Today my clutch got soft, checked reservoir and it was low on fluid (Dot 3) so I topped her off to the correct level.
Clutch peddle tightened up, I felt good.

Clutch does not seem to be slipping but I am loosing peddle and fluid.
Don't see any leak from Bell housing area or on wheels (brakes).

Did notice some ever so slight bit of moisture on Booster, but not the amount of fluid that I seem to have lost.

When I got back from about a 100 mile trip that took me through some stop and go traffic, I used the clutch between 30 to 50 times on the trip back, checked fluid level and I had lost about 1/2 of what I had filled to level.

Question 1: could this be the Master Cylinder getting bad?

Question 2: If it is the Master Cylinder can I switch the one in my '77 to the '80?

I have posted two pictures there are two wires that I assume one is a low level sensor and the other ABS, but I do not know for sure.
1977 1980

As you can see there is a wire attached to each reservoir on both there is a 2nd wire.
In the '77 there is a silver colored sensor on the 80 there is not.

My plan is to switch the Master Cylinder's and the connections.

Last Question Do you think this would solve the leak and clutch problems?

Any help from the goto guy's would be appreciated.

__________________
Collector of Fine Hose Clamps
'77 240D 'AVA' is in heaven now
'80 240D Kanarienvogel
'82 300TD 343k was my daily driver 'ADOLPH' In Surgery for a severe Deer Bite to the Nose
'88 560SL 102k 'White One'

Pull you drivers carpet up and see if the main cylinder is leaking.....usually its the slave that goes....but that one can too.....if the brake booster is wet....it could be leaking into the booster or it could the cloth hose going through the firewall..,

__________________

I don't have a MBZ with a clutch but they all work the same. If you see any fluid outside the master or slave cylinder the part is bad and needs to replaced or rebuilt.

Master cylinders frequently leak internally and won't hold pressure. Slave cylinders usually leak out the end. You may have to pull the rubber cup back to see the wet of leaking fluid.

I would do master and slave at the same time because they both frequently fail nearly together and you'll only be bleeding again when the 2nd one goes.

Rebuilding used to be sop. Cylinder hones can still be bought cheap.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

The Brake MC does not operate the clutch, that is not the Clutch MC. The Clutch MC is bolted to the Clutch Peddle assy. under the Dash.

The Brake MC Reservoir supplies fluid to the Clutch MC via a hose connected to the Nipple on the side of the Reservoir. The Hose goes in through a Grommet in the Fire Wall to the Clutch MC.

Lift up the carpet and see if the Clutch MC is leaking.

The rear section of the reservoir supplies fluid to the MC.

As mentioned, the fluid loss could be into the Brake Booster.

This video explains how it works. The MC is shown in a horizontal position with a reservoir cup. It is positioned in the MB in a vertical position, and the supply hose from the Brake MC connects in this spot on the MC.

BMW, VW, Porsche Clutch Bleeding - YouTube


Charlie

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

iirc the rear part of the reservoir also supplies the back brakes in the split circuit system so the leak could also be at the brake side of things and not the clutch. The second connector in the 80 pic is simply an additional level sensor for the split tank.

Been there, done that...
It sounds like the brake master cylinder is leaking into the booster = lowering the brake reservoir fluid level and exposing the brake reservoir clutch feed port to suck air = each fluid refill allows the clutch master feed line air to bleed out returning normal pedal.

Replace the master, and try to suck the brake fluid out of the booster.

Note: The large O-ring between master and booster is critical, many people forget to install it = massive vacuum leak...

.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/