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new master brake cylinder - pedal does not get firm

kronus

By kronus
in BMW 2002 and other '02

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kronus Community Regular

kronus

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A while ago I was having a problem where my brake pedal would go all the way to what I assume is the floor (it hard-stops after traveling) instead of firming up and stopping before that point. Bleeding didn't fix this. I put in a new master cylinder (for an E12 5 series), and am still having the same problem. I've bled the brakes three or four times now, and the pedal feel does not seem to be getting better at all.

Is it possible that my master didn't fully bleed and the repeated bleeding since then hasn't taken care of it?

Is it possible that my brake lines are expanding so much that the pedal goes to "the floor"?

Any suggestions? The current situation doesn't feel extremely safe. Interestingly (and I'm not sure if this is a possible clue), the pedal will firm up real nice if I pump it a few times with the engine off, but it will remain soft with the engine on. I am not losing fluid into the brake booster as far as I can tell.

Dmitriy '74 Fjord 02, '78 635csi

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c.d.iesel Grand Master

c.d.iesel

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....is it possible that my master didn't

fully bleed and the repeated bleeding

since then hasn't taken care of it? = YES , use a pressure bleeder

and flush the clutch circuit at the same time

why would you use a 5-series master cylinder on a 2002 ????

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

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kronus Community Regular

kronus

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....is it possible that my master didn't

fully bleed and the repeated bleeding

since then hasn't taken care of it? = YES , use a pressure bleeder

and flush the clutch circuit at the same time

why would you use a 5-series master cylinder on a 2002 ????

The piston diameter is a fair bit bigger than the stock 2002 one, which, if i could get the system bled, would lead to a stiffer pedal and higher line pressure, desirable for the pending volvo/320 brake upgrade.

Thanks for the advice, folks. I will dig up a pressure bleeder and have another go at it.

Another post in another thread mentioned reverse bleeding the master by hooking the clutch line up to the FR caliper. Thoughts about this method? Not sure if it will work right for dual-circuit brakes....

I was saving my stainless lines for when the bbk stuff got here, but i guess i will put them on to factor out possible problems there.

Dmitriy '74 Fjord 02, '78 635csi

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nbristow01 Grand Master

nbristow01

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....is it possible that my master didn't

fully bleed and the repeated bleeding

since then hasn't taken care of it? = YES , use a pressure bleeder

and flush the clutch circuit at the same time

why would you use a 5-series master cylinder on a 2002 ????

The piston diameter is a fair bit bigger than the stock 2002 one, which, if i could get the system bled, would lead to a stiffer pedal and higher line pressure, desirable for the pending volvo/320 brake upgrade.

Thanks for the advice, folks. I will dig up a pressure bleeder and have another go at it.

Another post in another thread mentioned reverse bleeding the master by hooking the clutch line up to the FR caliper. Thoughts about this method? Not sure if it will work right for dual-circuit brakes....

I was saving my stainless lines for when the bbk stuff got here, but i guess i will put them on to factor out possible problems there.

Several guys locally have put the 5 series MCs on their cars and say it is a nice upgrade. What size tires are you running? The added mass of larger tires and wheels can cause the same issue from what I have read. Can overpower the shoes and pads of the stock setup? Like I said I read that on FAQ but have no working knowledge of it

I'm not as dumb as I look

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kronus Community Regular

kronus

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Several guys locally have put the 5 series MCs on their cars and say it is a nice upgrade. What size tires are you running? The added mass of larger tires and wheels can cause the same issue from what I have read. Can overpower the shoes and pads of the stock setup? Like I said I read that on FAQ but have no working knowledge of it

I'm running 205/50/15 r888s, which are substantially larger and stickier than the stock wheels/tires, but this should not affect pedal feel. A properly bled system should firm up and stop before the pedal reaches the mechanical limit, regardless of the tire situation.

Dmitriy '74 Fjord 02, '78 635csi

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allbim Mentor

allbim

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For a given amount of pedal force, a larger piston in the master cylinder will give you less, not more hydraulic pressure; and less braking force than a smaller piston.

Take a look at the pump-piston on your hydraulic jack; it is very small for a reason.

A hydraulic brake system is just like a mechanical lever. Your foot operates the long end of the lever and the short end operates the brake pads and shoes.

The greater the the brake-pedal movement, compared to the caliper-piston movement, the more force is developed at the caliper.

Yes, a bigger master cylinder will give you a stiffer (firmer) pedal, but it you will have to step on the pedal harder to make the car stop. Bigger is not always better.

More on the same subject http://www.physlink.com/education/askexperts/ae526.cfm

No amount of skill or education will ever replace dumb luck
1971 2002 (much modified rocket),  1987 635CSI (beauty),  

2000 323i,  1996 Silverado Pickup (very useful)

Too many cars.

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kronus Community Regular

kronus

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For a given amount of pedal force, a larger piston in the master cylinder will give you less, not more hydraulic pressure; and less braking force than a smaller piston.

Take a look at the pump-piston on your hydraulic jack; it is very small for a reason.

A hydraulic brake system is just like a mechanical lever. Your foot operates the long end of the lever and the short end operates the brake pads and shoes.

The greater the the brake-pedal movement, compared to the caliper-piston movement, the more force is developed at the caliper.

Yes, a bigger master cylinder will give you a stiffer (firmer) pedal, but it you will have to step on the pedal harder to make the car stop. Bigger is not always better.

More on the same subject http://www.physlink.com/education/askexperts/ae526.cfm

Hmm, interesting point. You're right... It feels stiffer because it needs to move less to displace the same amount of volume, but the piston is larger and produces less line pressure for equal brake pedal pressure. I guess the main benefits are the pedal feel and better control of threshold braking.

Dmitriy '74 Fjord 02, '78 635csi

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