W123 quest for a high firm (rock solid) brake pedal - PeachParts Mercedes-Benz Forum

Rocket99 · #1 01-22-2017, 11:00 AM

FSM : on checking brake booster page says:
Overlap between push rod piston of main cylinder and push rod of brake booster:
0.2 to 1.2 mm.
this means push rod on brake pedal/booster moves the main cylinder piston by that much , at rest, ie before pushing the brake pedal, right?

Mine has unsatisfactory gap / slop, so the pedal requires travel before it begins braking.

Now it's drivable but I would NOT take it on a long trip or even beyond local in town driving

After:
new ATE master,
endless bench and caliper bleeding (ad nauseum)
all new hoses
rebuild of rear calipers

still the pedal is:
variably firm, still soft enough for concern
supports only one or two pedal pushing before loosing power braking

vacuum pump:
delivers steady 20 in. Hg at idle and at revving with gauge directly on its output
vacuum consumers side branches deliver only 1 to 2 in. Hg at idle and 7 at revving
the booster sees no more than 12 in. Hg at revving with gauge on the disconnected from booster check valve tube

t walgamuth · #2 01-22-2017, 11:04 AM

Are you an experienced brake bleeder?

Just to recap (sorry if I assume you know less than you do) ...pump pedal until you get some firmness, hold the pedal against that firmness while a partner opens the bleeder enough to let your pedal go down to floor without letting it up until the bleeder is closed. Then repeat until you cannot pump the pressure point any higher. Start with the furthest wheel from the master and do each wheel closer in order.

Good luck!

funola · #3 01-22-2017, 11:32 AM

Some important details:

Put a tight fitting clear PVC tubing on the bleed nipple and run it upwards then down into a clear 12 oz water bottle. Open the nipple a crack, just enough to let brake fluid and air bubbles to escape. You'll clearly see bubbles rising with each bleed. When you see no more bubbles, you are done bleeding. Because the tube is going upwards and always full with brake fluid, it prevents air from being sucked back in.

I never use a helper. My helper is a wooden stick jammed between the pedal and the steering wheel. Pump the pedal easy and steady (I do approx 10 strokes each time), not hard and furious to the floor like in a panic stop.

Rocket99 · #4 01-22-2017, 12:07 PM

Thank you to both of you.

Now there is a new finding: Cannot get the booster to pump up at all: zero in. Hg, using a hand held vacuum pump. No wonder the vacuum assisted after one or two strokes, braking is "spent". Verified the seal between MC and booster is present. So now I'm trying to determine the best brand of brake booster to get.

I've been using motive pressure bleeder exhaustively, put maybe 3 full quarts thru the caliper bleeding routine. I'm somewhat rural and ordinarily have no helper on hand.

architecture man: Yes sufficient past experience, have used your technique over the years. In absense of helper readily available I'm confident pressure bleed is a suitable substitute. However given the perpexing sustained probleme when the opportunity arises I'll give that a try too if I do not reach satisfaction.

Funola: also have observed caliper bleed product for bubbles thru clear tubing, even catch the product and filter it for re use until the major issues are resolved, ie achieving a high rock solid pedal like the one I had before this repair. When its working correctly I'll do one last bleed with virgin fluid.

This process started as replace a trailing arm that broke right in half. The brake repair is a detail I discovered needed doing when almost back on the road at the end of the trailing arm saga.

I have used your exact idea of "the stick" and after a similarly frustrating brake repair on "96 Dodge Ram 2500 cummins involving prolonged frustrating lack of success that ended up in a very good result, but I could not tell you why. The stick was part of it . Speed bleeders were part of it.

Thanks again guys

funola · #5 01-22-2017, 12:37 PM

There is an o-ring between the MC and booster, did you replace it with a new one? Did you lose the old one. It will not hold vac w/o it.

Rocket99 · #6 01-22-2017, 01:32 PM

Quote:

Originally Posted by funola

There is an o-ring between the MC and booster, did you replace it with a new one? Did you lose the old one. It will not hold vac w/o it.

new mc has one in place

thx

vstech · #7 01-22-2017, 02:55 PM

Sounds like you need to fix or bypass all vacuum consumers before tracking down the booster issue...

A handheld mityvac is fairly useless for pumping up a booster, but it should show SOME vacuum after a few pumps...
With the vacuum pump dedicated to the booster, how much vacuum can you build? Should be able to achieve full 20"...

Rocket99 · #8 01-22-2017, 04:49 PM

The test showing max 12 in. vacuum available to booster was with consumers plugged off and also plugged in as normal. No difference, they are not getting any vacuum to speak of.

I need to replace the vacuum check valve / tube since its delivering only half the vacuum to the booster and 1 in. to 7 in. Hg to consumers. I'll need competent vacuum to repair those functions and they could work fine now .... wirh sufficient vacuum.

If brake pedal chachteristics improve maybe I'm done

Now there is 2 in, of pedal motion before braking begins. Its like I need an adjustable booster rod to erase 1.5 of the 2 in of ineffective pedal motion. FSM says have booster rod press on MC piston causing .2 to 1.2 mm depression of MC piston at rest.

Braking power assist drops off fast when you pump the pedal a few times.

Need a witch doctor.

Diseasel300 · #9 01-22-2017, 06:11 PM

If you have 2" of pedal travel before you get any braking action, stop looking at the booster, you have air in the system somewhere.

You very well could have a booster issue as well, but that generally results in a lack of power assist - read: excessively firm/stiff brake pedal requiring excessive effort to get braking power.

Tackle one thing at a time. Get the braking system bled first. Without a working booster (assume NO vacuum) you should have a really hard brake pedal and it should take a fair bit of effort to push it.

Once you get that sorted, focus on vacuum and booster issues.

Rocket99 · #10 01-22-2017, 07:46 PM

vacuum supply is my next priority, insufficient boost could be dangerous especially if its deteriorating, as it may be Booster is not getting enough vacuum to do its job right

Its safe enought to be drivable with the excess pedal travel before brakng starts.

I have no doubt bleeding issues are also in line after that ... and perhaps replace booster

Thanks

Diseasel300 · #11 01-22-2017, 09:27 PM

You have things backwards.

A sinking/long-travel pedal means you have excessive air in the line. Because the air compresses, you CANNOT develop full braking pressure. The booster has nothing to do with this.

If the booster is leaking or out of the circuit entirely, you will have a hard brake pedal. You can still stop the car like this, but it takes extra effort. It is designed so that if you lose vacuum or the booster fails that you can still safely stop the car.

Get the air out of the system FIRST. With the engine stopped and the vacuum bled from the booster, your brake pedal should be nearly rock hard. It shouldn't take much -if any- travel to encounter significant resistance. If it is spongy or takes the 2" you claim it does to get startlingly firm, you have more bleeding to do.

Fix the air in the system FIRST. It is a serious safety issue!

Rocket99 · #12 01-23-2017, 09:17 AM

Pedal is not sinking, but does not apply any braking until 2 in. of pedal travel.

First thing in the morning the pedal is high and quite firm, seems the booster is not providing assist at that point, ie it is leaking vacuum

shortly after starting it, beginning to drive, it still provides good braking, locks 'em up with a strong foot, , quite firm, perhaps as firm as fully functioning, only very slightly spongy at its "top" ie when it starts braking 2 in. down

the vac tube / check valve furnishes max only 12 in. vacuum, while reving , 7 in at idle (below FSM specifictions, its old, clogged apparently, furnishes near zero vacuum to other consumers

The booster may be leaking, it is impossible to generate any vacuum at all on a gauge with hand held vacuum pump, so it seems to be leaking, MC oring is present

Since renewing that tube is a quick easy ,

Thing is: I must have run 2 to 3 quarts of B. fluid thru using pressure bleed, bench bled repeatedly and got sinking pedal over and over. Finally the pedal no longer sinks slowly to the floor, (MC finally bled right?) braking is good, safe to drive though must get pedal back to high and firm

Are you suggesting: air is in MC or in lines to calipers??

Thank you for your responses.

I realize the picture I paint is limited by my knowledge. I have worked on 'em for years since quite young, but only as a hobby that keeps me away from repair shops

t walgamuth · #13 01-23-2017, 10:13 AM

Sorry to ask but the rear reservor in your mc is not dry is it?

Rocket99 · #14 01-23-2017, 10:18 AM

no, I see the point though

I notice the first 1/2 in. for MC piston travel applies brakes to fronts only .... then only to the rears

I realize I'm so close to this, and coming from my own limited knowledge I may not furnish good baseline information.

Thankfully its OK to drive around town ..... they are locking up with a hard foot on the pedal

the LF caliper may be dragging a little., only a little

Thx

Diseasel300 · #15 01-23-2017, 01:26 PM

I think you're still missing the point. Cold, hot, driven, or parked, the pedal shouldn't change it's behavior. The booster will make it FEEL easier to press, but you should still get a reaction out of the brakes at the same pedal travel - booster or not.

Again - a failed booster will result in a HARD pedal, not a spongy one.