sticky brake booster ?

[ashley]

I have a 68 280 SL 4spd, its nearing the end of a cosmetic restoation. During the first test drive the brakes were dragging. Figureing the hoses were constricted I replaced them all and sent the calipers off to the rebuilder. I then blew out all the lines and drained the MC, reassembeled, and replaced fluid with synthetic. Now the brakes are fine for 5 mins or so untill the car warms up, then they hang up. I've traced the problem to the rod that enters the booster, when the pedal is released the rod fails to return the last 1/4 inch. I can pull it easily with my fingers and no more hang up. The pedal returns fully, but the rod does not. Its all been lubed but why does it hang up only when its warm? The vacum line is new as well. I'm trying to aviod removing the booster and MC as they seem to be fine. I've tied a string to the end of the rod for now which I can tug back from the drivers seat, up and over the windshield, most inelegant, but I'm still working out the bugs. Any thoughts?

[Benz Dr.]

You have to match the master cylinder to the brake booster.

[jacovdw]

Firstly, as Dan already said, the master cylinder needs to be the correct one for your pagoda.

Secondly, assuming the booster has not been disassembled and rebuilt, there is an adjustment nut at the rear of the booster that needs to be properly adjusted (see attached image).
The bolt that you see circled in red is the one that connects the pedal to the booster and is an eccentric bolt and if improperly adjusted could be an explanation for your symptoms.

On one side you will see a notch cut into the head. The bolt should be adjusted so that the notch should be pointing to the rear of the vehicle.

[ashley]

Thanks for the response. The master and booster never were removed so they are original to the car. The adjustment bolt was never disturbed, however I did loosen it and spin it around to see its range of adjustment, it does not seem to be concentric, just a uniform bolt. I think I read that the 69 did not have a cocentric bolt.

[jacovdw]

Ashley,

Your car should have the eccentric bolt and it was used on all pagodas.
Much later Mercedes eliminated the need for the eccentric bolt by adding a plastic bushing that covers the cam of the bolt thus eliminating any adjustment.

Does the bolt on your car have a notch in the head?

If so, then the notch points to the highest point of eccentricity of the bolt. There is not an awful lot of travel visible to the eye when you adjust the bolt though.
Another option would be to remove said bolt and inspect it and it's sleeve closely.

[ashley]

So, yes I do have bolt with a notch on the head and it is concentric (eccentric?), by rotating it wouldnt it just change the position of the rod that goes into the booster and perhaps the amount of pedal travel? My feeling is that there may be a rusted spot on the rod from sitting unmoved in one place for 15 years causing it to hang up a bit, and that by turning the bolt 180 it would move the rusted spot a bit allowing it to slide on a different section. No?

[jacovdw]

Rotating the bolt does not alter the position of the pushrod that goes into the booster in any way, but rather has to do with pedal travel.

In any event, the bolt should be clean (as well as the associated area) and lubricated to avoid binding.
If you look closely at the whole mechanism, you would also note a pivot point right at the bottom (below where the rod from the pedal assembly pushes against the lever that operates the booster rod).

That pivot bushing can be removed and cleaned as well and any rust/dirt/grime can prevent smooth operation.

I would recommend that you look at this first and clean/lubricate the assembly  and adjust it (notch facing the rear) before we look at the booster.

[ashley]

It does seem that it was the lower pivot that was binding up. Thanks for the advise jacovdw, I removed the concentric bolt and worked the arm back and forth while spraying the lower pivot with pb blast and it slowly got easier to move. I  still dont know what the temp of the engine had to do with it, but anyway it seems to be ok. Thanks again.

[jacovdw]

Glad I could help.

My theory regarding the engine temp is as follows: increase in engine temp = increase in temperature under the hood (bonnet).
The resulting heat caused the already gummed up parts in question to expand and bind, thus the "hanging" brakes.

[Bonanza]

I just now posted to the Need booster thread and pointed out the pivot bolt issue. On my car (66 230SL) I had to remove the new booster and then remove the pivot arm. I took the arm to a machine shop and had them press out the bolt. I tried to press it out myself but no go. After being where it was for 45 years it was not going to give up without a fight. Its all good now.  Good Luck