drum brakes master cylinder with disc brakes?

[GGR]

I've read many times that the brake master cylinder needs to be replaced when one swaps rear ends and goes from drum to brake discs. I took apart a 230 master cylinder the other day and the only difference I could see with the disk brakes master cylinder is a pressure valve fitted in the port for the rear brakes. Other than that, internal diameter etc. is the same, as per the workshop manual. So, can we assume that by just removing the pressure valve inside the port for the rear brakes circuit would be sufficient to have the master cylinder work fine with disk brakes at the rear?

It is interesting to note that early disc brake systems also had a pressure valve (but different from the one for the drum brakes) fitted into the port for the rear circuit. I'm talking here of later circuits as found on the W108 4.5 as these are the main rear axle donors for the Pagodas.

[jameshoward]

I don't know about ten differences, but everyone spoke to (ie, pagoda ninjas) told me that changing the master cylinder to the correct disc MC was the right way to do it. It wasn't hugely expensive, and was easy to get.came from MB in 24h for 120 euros.

[garymand]

So you put in a disc brake rear axil, did you include the pressure regulator at the rear of the car too?  I think it limits the pressure so the back doesn't lock up early on hard braking.  The seals to wear and can break through soif you had the master apart a rebuild including bore inspection would have been good.  On the other hand, a new master isn't a bad investment at a very reasonable cost.

[jameshoward]

Hi Gary,

Yes, I put in a proportioning valve, a brand new one from about 30 years ago thanks to a super star on the forum. Naj mentored me through the swap but it was pretty straight forward. I was lucky enough to get one of German's leading restorers od 113s to go over the car once I had finished the work. He said most people fail to do the two key things you've alighted on (I was secretly proud). He then moved the valve about 2" to make it correct!

Anyway, my conversion is by the book.

James

[Benz Dr.]

When I changed to the LSD on my 230SL I kept the same booster with the master cylinder and I haven't hit any trees yet. Seems like it works just fine.