I just put in a brand new master cylinder today and bled out all the lines. The pedal is a good bit softer than it was before the new master cylinder. Before I get under there an bleed them all again, is there anything else I need to check?

Here's a rundown of recent brake related repairs/problems.

• mid may - brakes seized up on highway and ruined the rear rotors, calipers, and pads
• mid may - replace rotors, calipers, flexible(rear) lines and pads. Bled the system.
• mid may until now- Brakes performing normally with normal pedal stiffness.
• this week- Brakes seized up again on highway but slowed down much sooner to save new components. (I hope!)
• Install new master cylinder, bled lines.

__________________

"Gerwalt" 1984 - 300TD

Re-bleeding the Mastercylinder followed by the rest of the brakes is the best you can do.
If the problem continues it has to be the New Mastercylinder (I had a rebuilt one that never performed well until 1-1/2 years later the pedal went to the floor). Or there is something going on in the part of the Brakes you did not work on.

I suppose there could also be crap in the Brake Fluid if previously the old Fluid was not flushed out.

__________________
84 300D, 82 Volvo 244Gl Diesel

Wait, what involves bleeding the master cylinder? This is my first time at this so maybe that's what I missed. I just bled the system at each caliper but didn't do anything to the master cylinder other than bolt it on. The Mercedes Service manual I have didn't mention anything about bleeding the cylinder.

__________________

"Gerwalt" 1984 - 300TD

The new master cylinder should have come with plastic plugs with nipples on them. You screw these into the outlet holes and place hoses over them, usually run back into the filler cap hole. The master cylinder is then blead by filling the resevoir and pumping until no air comes out of these lines. This gets any air trapped in the pistons and check valves out. Then the break lines are attached and each cylinder blead.

__________________
1992 300D 305+K miles
1978 240D Sold

Hmm, we will be ordering one soon, I will have to see if it comes packaged this way. The only one I have ever replaced was on our (now long dead/gone) dodge caravan, and it did not come with, nor require, such a procedure. I didn't do it to our 300D/and two SD's either when re-doing the brake lines. (But did not re-do the master cylinder.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

This is a site with pics that I found fast searching + bleeding a mastercylinder.
Your Mastercylinder does not look like his but the Idea with the tubing is the same with the exception that most people stic the tubes in the Brake fluid Resivoir.

The main idea is to keep the ends of the tubes under the Brake Fluid so Air does not get sucked back in through the tubes. Also you do not have to put it in a vice to do it. Just leave it on the attached.
Some Members have suggested having the Engine runing when pressing on the Brake Pedal during the Bleeding process to not stress the old Diaphragm in the Brake Booster.

http://www.tegger.com/hondafaq/mastercylinderreplace/benchbleeding.html

After that it is still a good idea to bleed all of the Brakes as you did previously to get out any random Air Bubbles and flush our any nasty old Fluid.

__________________
84 300D, 82 Volvo 244Gl Diesel

It's always a good idea to bench bleed a new MC before installing the lines and bleeding downstream. Sounds like that's the step that wasn't done.

steve a

__________________
Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

Almost every manual says to bench bleed the MC though I've only found it necessary on a couple of vehicles. When I replaced the MC on my current '92 I couldn't get fluid to flow to the rear without bleeding the MC first. My old 240 didn't need it, but my neighbor's Camry did. Fussy air bubbles! One of those things that it doesn't hurt to do.

__________________
1992 300D 305+K miles
1978 240D Sold

Before you rip everything out I've read here - on this forum - that wedging the brakes on each night before you go to bed (or when you're not using the car) helps with stubborn bleeding.

I've had similar problems with my brakes and it does improve matters.

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I have always been hopeless at bleeding brakes.
If I can get to the stage where with full pressure on the peddle its not on the floor, its good enough for me!!
Over a few weeks they normally firm up.
You are all welcome to call me an i##ot if you wish, but it works for me.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Another thing I've just found out reading through the FSM - the sealing ring between the master cylinder and the brake booster should be renewed. If you've been having vacuum problems as well as the brake problems this should be something you should take a look at before replacing either a master cylinder or a brake booster.

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

+1
That rubber ring is the seal for the booster and nothing else will run right with a vacuum leak either.

__________________
1992 300D 305+K miles
1978 240D Sold

The MC I bought was new and it had a new gasket on the end of it. There were plastic plugs in the holes but they were flush with no nipple. I'm going to try loosening the hard lines out and see if I can get some air to come out. Then bleed the rest of the lines again.

__________________

"Gerwalt" 1984 - 300TD

If I were you I wouldn't loosen anything at the master cylinder. If you do that I think you are more likely to get air back into the master cylinder... Just bleed round the car again from the caliper bleed nipples.

The FSM says that you should pump the brake pedal at least 3 times per bleed nipple when you are installing a new master cylinder.

To my mind that says it is perhaps better to get some help bleeding when you've got this stubborn sort of problem. One person on the brake pedal - the other lying in the dirt shouting "UP" / "DOWN"!

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Speed bleeders are the bomb.