740 GL: How to adjust brake booster?

Its a 740 GL, 1989, brakes are non-ABS.

The brake padal doesn't feel spongy like air but you have to go all
the way to the floor, or nearly, before the car will stop. I assume
the brake pads are still good, there are no tell-tale metallic noises.
When bleeding the brakes estensively did not help, I changed the brake
booster; then when that did not fix the problem, I changed the master
cylinder. Bled the brakes extensively again....same thing!

It was this way when I first got it, but I am thinking that it is
like my 85 mercedes...you shouldn't have to go more than a quarter
inch even, before you begin to brake the car.

IT looks like the only thing left is the adjustment of the booster.
Don't have any manual to explain how to adjust it. To get it to brake
earlier (with less travel). I would think it has to start from a more
forward position. Do I turn the pushrod, or the clevis connected to
the brake pedal? IN what direction?
Before I atttached the new master cylinder, I had someone push on
the brake pedal with engine running while I watched to see how much
the pushrod was moving. It moved forward about an inch or inch and a
quarter, I would estimate.
Thanks for your help.

 

There's no adjustment, whatever it is you're looking at, resist the urge to
fiddle with it and leave it alone. Either you have air in the system or the
master cylinder is bad.

 

IIRC the non ABS models have to be bled in a certain order,as otherwise
the system assumes fluid loss on one brake line and a shuttle valve
cuts of the fluid to that circuit.This leaves you with working brakes
on one front wheel and the opposite wheel at the back and results in
increased pedal travel.The way to cure it is to bleed a wheel on the
working circuit,this lets the shuttle valve centralise,thus restoring
full brakes and less pedal travel.I can`t remember the exact details
but I have a manual somewhere and will look at it tomorrow if I have
time.
regards,Mark.

 

Funny that I am told there is no adjustment. I remember when I
installed the new booster that it mentioned a factory setting and
something about possibly needing to change it, am pretty sure it
involved rotating the yoke which connects to the brake pedal. Lost
the little installation instruction sheet that came with
it...otherwise I would re-check that. I don't even know how far away
from the shuttle valve the front end of the pushrod is with brake not
depressed. But if it has to move forward a half inch before even
touching it, then obviously the car is going to have a lot of brake
travel!

Anything further you can tell me on getting the brakes properly
bled would be appreciated.

There was a "benck-bleed" procedure given for installation of the new
master cylinder. I did it exactly as stated, got the master cylinder
completely full/bled. But it was a waste of time! Reason is that
obviously I have to take the two plugs off the botttom ports and the
two fittings with tubing off the other two ports before I can
reconnect the brake lines...and so flud leaks out massively and air
comes in!

Guess I will jack the car in the morning to see if one half of the
brakes is not working.

Thanks all!

 

It sounds like you've got the same problem I had about a month ago. In
my case, the master cylinder had given up the ghost. A replacment from
the local scrappy cost me £20 and about an hour to strip out the old
one, flush the system and bleed.

Hope that helps,

Richard

 

If you can find a recently replaced one at a scrapyard that's not a bad way
to go, though given the effort to replace it and bleed the brakes, and how
critical it is for it to be working I'd be inclined to go with a new one.

 

but he said he changed it already

 

The shuttle valve I`m referring to is not part of the servo/master
cylinder.It is a seperate small block mounted lower down the bulkhead.I
forgot to look for the manual.If this is the problem it would be easy
to prove by jacking the back of the car up,having a helper press the
pedal and try turning the wheels.One of them should not brake.
regards,Mark.

 

Is there any wear in the pedal where the pin goes through the pushrod?I
had that once on a 1985 240 the hole in the pedal had worn to a slot.
regards,Mark.

 

Finally remembered to look up manual.It says,and I quote "If a new
servo (booster) is being fitted,adjust the pushrod if necessary to give
a small clearance between the servo pushrod and the master cylinder
piston in the resting position"
Easy when you have the book.
regards,Mark.

 

After the booster is adjusted, you can expect the original problem to
remain.

A "sleeper" is the lubrication of the front calipers. I let mine go too long
(on an '85) and the brake pedal kept going lower. In addition, the car
pulled to the left when I put on the brakes. With the front wheels off the
ground I found the right wheel could still be turned freely when the pedal
was floored - not a good sign. The slide pin had seized and would not let
the caliper float, so one pad was several mm away from the rotor. The rear
brakes (at least on mine) are dual piston and don't have that issue.

Others have reported the brake hoses (going to the calipers) deteriorating
and causing symptoms like this, but probably with more gradual onset.

Mike