Need Help with 25mm MC and Brake Proportioning

[Slowered318]

Calling out for someone with brake system expertise.

I made up a diagram explaining my planned setup. I have all new parts so I can't really turn back now. My rotors and callipers are stock size, I installed new pads last year, master cylinder is almost shot and I need a complete brake fluid flush. The car has stock wheels with good rubber.

What prompted me to do this. My pedal is soft and not very responsive, stopping distance is rather poor. When my ABS kicks in (only wet roads) I can hardly force the car into a skid and the pedal sometimes sinks almost to the floor. Also when threshold braking I think the fronts are locking up before the rears. I think the stock bias is a little too idiot proof for me as I know how to drive this cars at the limit.

My question is that if I remove the stock pressure regulator and install an adjustable one will it give me better control over my brake bias and help me to use all 4 wheels to stop? Also if the master cylinder is 25/25mm bore compared to the stock 22/17mm, will this cause any problems because of the ABS like excessive sensitivity? Will having a restriction on the rear line cause any problems to the master cylinder, blow by or rupture the seals somewhere?

Sorry so many questions, I've searched for days/weeks and can find any real details. Is this all a really bad idea? I don't want to spend thousands on a big brake kit and new wheels/tires, just want to tweak and tune with the original brakes.

[bmwman91]

I tried swapping in some 25/25mm master cylinder many many years ago and all it did was cause my rear wheels to lock. So yeah, you would need a bias adjuster.

If the brake pedal feel sucks then there is probably air in the system. I'd recommend bleeding the hell out of it and installing stainless braided lines. That makes a big difference in pedal feel. Swapping in a bigger MC and playing with the bias is another solution, but if there is air in there then you still need to handle it. Stainless lines are a must in all cases anyway!

[Warsteiner]

+1 for what bmwman91 said.... I did it on my E30M3 race car with Wilwoods but decided to get my stock system on the 318 working well. It really is a great system for the street if you bleed thoroughly and have good pads. I cant remember if I run the Hawk HPS's or the HP Plus. LOL

Cheers,
~Ralph

[Slowered318]

Don't worry, I have stainless steel high pressure lines already in there. So Ralph and bmwman91, did you both find the rear brakes would lock up? Did either of you try a proportioning valve or has anyone removed the factory pressure regulator?

[Warsteiner]

I did not do it on my 318 because is was not worth the effort for continued use of the stock calipers. It just didn't make any sense to me. There's a little bit of brake line work that needs to be done in order to change over the MC. It's not a PnP. You definitely need to remove the stock proportioning valve. It can be done but for a stock system, I didn't want to chance what could happen to the balance of the car when braking by doing such a radical change since I wasn't putting in a new proportioning valve. 22/17 to 25/25 is definitely a change....

If you're really worried about the pedal feel then I would start looking at the calipers and make sure they are bleeding properly. If you haven't changed over to a different color (blue to yellow or vice versa) in brake fluid yet, now is a great time to make sure all the old is out along with any condensation and air in the lines. You should get a great pedal feel if all is cleared out. If not, then maybe the MC does need some attention.

As for locking up the back brakes.....yes I have done it on my M3 at Watkins Glen after the uphill coming into turn 8 at 100mph slowing to about 80.....locking up....letting off and reapplying brakes to only end up sliding sideways and spinning through the apex to end up facing oncoming traffic at track out. It was a good thing I knew who was behind me (in the instructor group) cuz I knew they wouldn't hit me....LOL  It wasn't fun in a controlled environment, so I can't even imagine parked cars, telephone poles, houses, etc..... Don't want to do that again but it does make you rely on your car control skills!

Cheers,
~Ralph

[Slowered318]

Fair enough, I see your point about the brake balance. If you upgraded the front callipers and kept the stock rears then you might have even more initial rear bias, especially before the Willwood's got enough fluid to bite in. Do you think an adjustable valve like the one I bought would have helped in your case, did anyone suggest that after your spinout at Watkins Glen, what made you remove the stock pressure valve?

Strange that so many tuners and people of the forums have changed to the 25/25 and recommend it. I have asked my mechanic and he said it's not a great idea unless I want to spend money and hours tweaking the brake system. On the M3 I think you need to change one of the lines going to the ABS unit, however on 318/325 is a strait swap.

As for brake bleeding. I have heard there is a process they can do at the dealership that actuates the ABS pumps while bleeding the brakes and helps remove any air pockets. Does anyone have experience? I have always just done the "two man" job in the driveway (don't get any wise ideas about that)

[bmwman91]

Fro bleeding, assuming that there is no air in the ABS unit, I start at the rear passenger wheel, then rear driver's wheel, front passenger and then front driver. If there is air in the ABS unit, it ca be VERY difficult to remove and you may indeed need to go to the dealer to have them do their special procedure.

When I switched to a bigger MC (was probably the same 25/25mm one) I thought that pedal feel did improve. However, having the rear wheels lock up was no good so I went back to stock.

[victell]

However, having the rear wheels lock up was no good so I went back to stock.

Why do you think the 25mm mc change made the rear lock up?  Did you keep the e30 proportioning valve?

[cristimm]

... You definitely need to remove the stock proportioning valve. It can be done but for a stock system, I didn't want to chance what could happen to the balance of the car when braking by doing such a radical change since I wasn't putting in a new proportioning valve. 22/17 to 25/25 is definitely a change....

Where is the stock proportioning valve located? Can you please post a realoem link with it?

[Slowered318]

Fro bleeding, assuming that there is no air in the ABS unit, I start at the rear passenger wheel, then rear driver's wheel, front passenger and then front driver. If there is air in the ABS unit, it ca be VERY difficult to remove and you may indeed need to go to the dealer to have them do their special procedure.

When I switched to a bigger MC (was probably the same 25/25mm one) I thought that pedal feel did improve. However, having the rear wheels lock up was no good so I went back to stock.

That's how I've always done it.

Strange that you didn't experience a change in pedal feel. I did a little math and volume calculations! For every mm of travel on the 25/25 master cylinder, you move 30% more brake fluid. Also the surface area of the original 22/17 master cylinder gives roughly 100% front and 60% rear. So in theory the adjustable brake pressure valve should be able to reduce the rear pressure enough to give me a stock brake bias, or even more front bias if I want. The difference being a more linear increase or decrease in pressure after the adjustable valve.

I'm sure it will do the job but my question is more complicated. I'm wondering what effect this will have on the master cylinder, will it equalize the front and rear pressure or will the restriction on the rear line cause it to "bind up" and stop me from being able to brake? And lets say I achieve the same bias as stock but I remove the proportioning valve in favour of a linear valve will I experience any problem with threshold or panic braking?

I found this useful http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake1.htm
The link might also explain why Ralph was having problems with the rear brakes locking up. Maybe because the larger Willwood callipers required so much fluid that the rear master cylinder got depressed. Just a thought?

Where is the stock proportioning valve located? Can you please post a realoem link with it?

It's below the brake booster and in the diagram on my first post.
http://www.realoem.com/bmw/showparts.do?model=AF93&mospid=47305&btnr=34_0153&hg=34&fg=15&hl=13

[victell]

Why do you think the 25mm mc change made the rear lock up?  Did you keep the e30 proportioning valve?

the original 22/17 master cylinder gives roughly 100% front and 60% rear.

thanks, Slowered318

[Warsteiner]

In my case it was probably too much pressure to the rear on my adjustable valve.

Cheers,
~Ralph

[cristimm]

It's below the brake booster and in the diagram on my first post.
http://www.realoem.com/bmw/showparts.do?model=AF93&mospid=47305&btnr=34_0153&hg=34&fg=15&hl=13

Thank you.

[Slowered318]

I hope I don't replace all this equipment just to find out the problem is a lazy calliper or ABS issue. I just can't seem to get the braking up to par with other cars I have driven, I even replaced the booster hose with a 300psi air line and installed a new check valve.