I noticed that my brake pedal on the 84 w123 travels pretty far before the brakes catch. I also noticed that the first half of pedal travel (master cyl) controls the front brakes and the second half the rear.

But here's what I was wondering...


As front brake pads get thinner and thinner (there's about 3/4" possible less thickness), the brake caliper pistons have a further distance to press in order to make the pads contact the rotors.

Does that equal more pedal travel or does the fluid somehow adjust itself to let the calipers have a new 'Zero point' for the worn brake pads?

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It should not equal more pressure or further pedal travel, as the calipers simply have more fluid in them and the pads are a little closer to the rotors, it compensates.

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Brake fluid somehow absorbs moisture in that seemingly sealed system. Brake fluid with water in it is more compressible than fresh brake fluid. Sounds like you're due for a brake fluid flush.

Sixto
87 300D

The "zero point" to which you refer is due to the distortion of the caliper seals when under pressure. As the pads wear, the piston will travel further, but it will always recess by the amount dictated by the seal distortion.

That is, if everything else is as it should be. Excessive wheel bearing play can cause the rotor to push the pads further in than necessary, which does give excessive pedal travel.

I beleive that increased brake pedal travel is an indicator that one of your two brake systems are INOP. I would check all 4 lines, and double check your brake fluid level.

The reservoir on a W123 is not sealed, it's vented.

Only when the water gets hot enough to boil.

I just changed the fluid, hoses, one front caliper and rebuilt the other. The brake pedal goes down much farther now than it did but the reservoir seems to be keeping fluid and the brakes seem to be working ok. I don't like the amount of travel though.

The rebuilt caliper I bought (O'reilly's Auto Parts) didn't have the brass piece that goes between the caliper piston and the brake pad but I'd doubt that'd have any effect.

I noticed one small hole in the seal on the caliper I rebuilt. But I didn't see any fluid leakage. I looked pretty closely at it and tried to get fluid to leak. I didn't notice any. I may have to change that caliper as well.

I don't think I'll rebuild calipers myself again. Too hard to get the seal on and the kit was $18 vs. a store bought caliper for $30 more.

But I'm wondering why the pedal goes down so far. It's all new fluid. I think I bled the brakes well enough. Maybe not.

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-E300d '99 350k
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-TDI Jetta '03 350k
Sold
-F250 '96 7.3
-Dodge Ram 12V
-E320 '95 200k
-E320 Wagon 1994 155k
-300d Turbo '87 187k miles
-E320 1994 200k
-300d Turbo '84 245k (sold to Dan62)
-300d Turbo '84 180k
-300sd '80 300k
-7.3 Powerstroke Diesel 15P Van 500k+ miles
-190d '89 Non Turbo 2.5 5cyl 240k (my first MB)
Tom's Imports of Columbia MO Ruined the IP in changing leaky delivery valve O-Rings - Refused to stand behind his work. Mid-MO MB drivers-AVOID Tom's.

When you installed the calipers, were the bleeder valves on the top or the bottom?

This was an ATE caliper and the bleeder valve is at the top.

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-300d Turbo '84 245k (sold to Dan62)
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-300sd '80 300k
-7.3 Powerstroke Diesel 15P Van 500k+ miles
-190d '89 Non Turbo 2.5 5cyl 240k (my first MB)
Tom's Imports of Columbia MO Ruined the IP in changing leaky delivery valve O-Rings - Refused to stand behind his work. Mid-MO MB drivers-AVOID Tom's.

That's good. Sometimes folks install the calipers on the wrong sides of the vehicle so that the bleeder valves are on the bottom; that makes successful bleeding impossible.

Perhaps bleeding the brakes one more time would be beneficial. Other than a little air in the lines, there is no reason you brakes should feel much different than before. Unless you happen to have some coincidental problem, like the master cylinder leaking internallly.

If your pedal travel increases during hard braking it could also be your booster going south.

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classic signs of air in the master cyllinder. either the MC is damaged, or it has not been bench bled. it could also easily be low fluid in the MC.

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