E32 740 Master Cyl problem?

[Kevin///m3]

Guys,

I recently had my whole brake system overhauled which included the very common E32 master cylinder upgrade (pn 34311156643).

I've found that the pedal takes much more strength to brake hard. Like so much I can't even engage ABS. Granted I do have sticky Dunlop Direzza tires.

Anyone else experience the same?

I'm running stock rotors, Stop Tech Street pads, steel braided lines, fresh fluid bled fully.

Thanks,

Kevin

[Broken88]

I bought the upgrade kit as well (came with the master cylinder and pre-bent new brake line to the ABS module) and I had this issue as well. After bleeding my brakes 10 times and going through everything, I discovered that the kit did not come with the o-ring that goes between the master cylinder and the brake booster. This was causing a huge vacuum leak there. I then just ordered a new e30 m3 o-ring and installed it. It resolved the vacuum leak and my brakes felt perfect.

[Kevin///m3]

I bought the upgrade kit as well (came with the master cylinder and pre-bent new brake line to the ABS module) and I had this issue as well. After bleeding my brakes 10 times and going through everything, I discovered that the kit did not come with the o-ring that goes between the master cylinder and the brake booster. This was causing a huge vacuum leak there. I then just ordered a new e30 m3 o-ring and installed it. It resolved the vacuum leak and my brakes felt perfect.

Wow, hopefully that's my issue to. It's like the brakes feel totally fine for any easy braking, but when you try to stop the car fast, just no bite at all.

You don't happen to have the part number for the o-ring, do you?

[Mick]

# 18

https://www.realoem.com/bmw/enUS/showparts?id=1003-USA-12-1987-E30-BMW-M3&diagId=34_0703

[Broken88]

Wow, hopefully that's my issue to. It's like the brakes feel totally fine for any easy braking, but when you try to stop the car fast, just no bite at all.

You don't happen to have the part number for the o-ring, do you?

Yes, exactly the same as my car. I would slam on the brakes, and it would stop, but I couldn't lock up the brakes. I would have to press the pedal super hard just to drive the car regularly. The 0-ring completely resolved the issue. I am interested to know if this is your problem as well. Let us know if this is your issue...

[Kevin///m3]

I just looked, my car has the o-ring. Wish it was such an easy fix. Maybe a vacuum leak somewhere else?

[Broken88]

Yeah its possible. I actually pulled the hose coming off the booster and blew in it. That's how I narrowed it down the the o-ring.

[Kevin///m3]

I've neglected to investigate this issue further and now that's she's put up for the winter, going to try and sort it out.

I borrowed a vaccum gauge from a buddy....where should I take the reading from and what values am I looking for?

Thx

[basketcase]

First thing I do when trying to diag a brake booster issue is easy and fast. If the car is running, turn it off. Then pump the brake pedal to expend any residual vacuum in booster, about 2 or 3 pumps. Pedal should be hard and up near top of travel when pressed.

Then with foot pressed on pedal, start the engine. The pedal should go down, that's how you know the booster is working.

If pedal stays the same, fault in the brake booster system.

[Kevin///m3]

First thing I do when trying to diag a brake booster issue is easy and fast. If the car is running, turn it off. Then pump the brake pedal to expend any residual vacuum in booster, about 2 or 3 pumps. Pedal should be hard and up near top of travel when pressed.

Then with foot pressed on pedal, start the engine. The pedal should go down, that's how you know the booster is working.

If pedal stays the same, fault in the brake booster system.

I actually did that test a while ago the pedal did go down.

Something tells me that I'm just getting partial boost and there's a leak somewhere.

[basketcase]

I actually did that test a while ago the pedal did go down.

Something tells me that I'm just getting partial boost and there's a leak somewhere.

If there is a leak somewhere, vacuum would dissipate over time and if left with engine off, when testing pedal it should be hard and near top of travel on first application.

[M3 Frank]

I don't normally come the site, so this is a "drive by" analysis.

I did the calculations comparing the two a LONG time ago; the OE 23.81mm diameter stock and the 25.4mm (correct?) big MC.

The results are that given a pedal pressure of 70#, and a Mu of .39, the OE translates to a front brake torque of 198 Lb.ft. and a rear of 118 Lb. ft. while the 25.4 MC produces 174# and 104# respectively.

At a pedal pressure of 100 # the OE produces a brake torque of 283Lb.ft. front 168 Lb. ft. the 25.4 MC produces 249 Lb.ft. and 129 Lb. ft. respectively with OE calipers.




the site just deleted my lengthy explanation!

I will leave it to you to figure out what proportionally greater pedal pressure you need to have to get the same as the OE M/C

So, this in my opinion is another myth to better performance, not engineering.

the fact is that with HP+ pads and decent tires you can achieve 1.2 to 1.4 negative G stopping power before the ABS kicks in with an OE system.

I'll leave it at that and look for the flames.

[mlytle]

you are correct that bigger MC is not a car performance improver. and yes, it takes more effort for the same amount of brake effect.

however....

bigger MC is not about stopping the car faster. it is about changing the feel of the brake pedal. it shortens the distance a driver needs to push the pedal for the same brake effect. this can improve the performance of the driver in managing the braking process.

of course this does not guarantee better driver performance, but it is another tool to adapt the car to the driver's technique.

[jimmy p.]

you are correct that bigger MC is not a car performance improver. and yes, it takes more effort for the same amount of brake effect.

however....

bigger MC is not about stopping the car faster. it is about changing the feel of the brake pedal. it shortens the distance a driver needs to push the pedal for the same brake effect. this can improve the performance of the driver in managing the braking process.

of course this does not guarantee better driver performance, but it is another tool to adapt the car to the driver's technique.

^^^^ This ^^^^
I use it because I like the rock hard brake pedal (I use it with no booster) it puts the pedal right where I want it and rock solid for rev matching (when I dont screw it up). I hate long brake pedals.
The ideal brake pedal for me is a load cell that is block of metal LOL.
I really like the feel of the larger MC vs the stocker thats why I use it. If you want more braking power you are barking up the wrong tree with the larger MC, you either need to alter the friction of the pads, the leverage of the size of the rotors or both.

[Digitalwave]

Jimmy summed it up. Also the increase in pedal effort is minor... I believe about 13% according to Dr. Gustave's site.