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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.

Reply to
Geronimo
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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.

Reply to
James Sweet

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.

Reply to
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!

Reply to
Geronimo

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

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

Reply to
Richard Setford

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.

Reply to
James Sweet

Reply to
torpainter

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.

Reply to
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.

Reply to
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.

Reply to
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

Reply to
Michael Pardee

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