Replaced Brake Booster- RPM Still Rises When Pressing Brakes

[Mike K]

As per my previous post, after redoing the entire brake system, I was still getting a rise in RPM @ idle when pressing the brake pedal.
https://www.sl113.org/forums/index.php?topic=25574.0

So today I replaced the Brake Booster with a new ATE T52, again fitted a new O ring between the brake cylinder and booster, fitted a new vacuum line and bled the system.

I’m still getting a rise in RPM when pressing the brake pedal 

Is there something I've missed? Any thoughts and input would be welcome.

Thanks and best,
Mike

[Tyler S]

How much of a rise are we talking about? Pumping the brake pedal will always cause a RPM fluctuation. Dont pump the pedal. During a press and hold A slight rise and fall of maybe 50 rpm or less is normal as the chamber equalizes. If the rpm rise does not return to normal after less than a second then there is a leak at the o-ring which you said was replaced, or the new booster is bad. It is all possible as I once got a bad one right out of the box. Some of these were made 40+ years ago and the rubber doesnt last forever.

[Thierry du Laos]

Or could it be an engine too rich???

[Mike K]

Thanks for the feedback Tyler!
It seems that my testing method was faulty- I was pumping the brake pedal and was seeing the rise in RPM.
By following your method i.e. press and hold the pedal, I get a small rise of approx. 50 RPM which immediately settles. All appears OK.

One thing to be aware of is that the new brake booster is slightly different to the old one:
The ends of the "fork" into which the brake linkage bolts into, have 2 equally sized holes.
The old one had one larger hole where the special bushing bolt slotted into. (see pics below).

There are 2 ways to deal with this:
1. Either order the metal bushing which fits into the  brake linkage and then use a normal 8mm bolt through the fork & linkage- http://www.sls-hh-shop.de/main/en/mercedes-230-280sl-w113-/42-brakes/43-mastercylinder/bushing-brake-linkage-if-booster-new-p-10952
or
2. I enlarged the 1 hole on the "fork" and used the original bushing bolt.

Best,
Mike

[Mike K]

Another modification I made, was to use the original thicker aluminium support plate from the old brake booster. The new metal plate just looks like cheaply manufactured crap and its colour doesn’t match anything in the engine bay.

It’s easy to do, the rubber bellows slips/pulls off the “fork”, swop the support plates and then slip the bellows back on  -pic 2 below.


Best,
Mike