Brake pressure regulator Options

[watsonrx13]

Eric's ad selling the 912 rear calipers started a discussion about the brake pressure regulator. I'd like to continue that discussion. Eric mentions that he wouldn't replace the regulator with a 'T' fitting. What I would like to know is how could you test the regulator off of the car? At $300 - $400 a piece from the dealer I would like to test mine before I purchase a new one.

Also, Eric mentioned that the front calipers shouldn't be used on the rear, even with the regulator. Is there any problem installing a set of rebuilt rears on and not installing the parking brake cables?

Any thoughts and suggestions are welcomed. BTW, I'd like to keep this discussion only for the 4-lug wheels.

Also, what are the thoughts about replacing the master cylinder with the larger one how does that effect the brake biasing?

-- Rob

[Eric_Shea]

I can't type all that crap again so I'll just paste it here: (IMG:style_emoticons/default/smile.gif)

You can also bias brakes with pad compound. ie: stickier pads up front and not so sticky in the rear...

That's not a great idea. It takes a LOT of hit or miss and experience with pad compounds to play this game. Dan is the only guy I know that has done this successfully and he races his 914 regularly in wheel to wheel competition. Do me (yourself) a favor, make a quick list of all the brake pads you know of for our 914's. Now list them, in order, from stickier to not so sticky. Ready... go! (IMG:style_emoticons/default/wink.gif)

The question is; why? When the proper caliper with the proper piston size for best biasing, made off the same casting, with the same pad size is available to bolt on to the same mounting location; why put an irregularly large pistoned caliper back there?

With any or all of the calaper combinations you discussed....do any of the combinations ALLOW the removal of the stock... brake bias/brake pressure requlator?...or do to a specific combination,is the B.B. P.requlator... removed to achieve maximum breaking ability?

Not on my car. Removing it does not acheive maximum braking ability.

The pressure regulator is SO WAY MISUNDERSTOOD by the 914 crowd. One day, back when we were smashing rocks together for fire sparks, someone said; "If you take the proportioning valve out you'll have better brakes." I bought into it for a while as well, until I did some further reading. There's a page dedicated to it in the factory manual and it's a great read. The funny thing is, the Alfa boys have the exact same valve and if you cruise posts on their board regarding the subject, they would think you were crazy if you wanted to take this out.

That said; the pressure regulator is not a proportioning valve or a bias valve of any sort so it stands to reason it has a purpose beyond balancing our braking bias (which the factory did extremely well with piston sizes... hence the chatting herein). There are two things that you should read to help you put this together in your own mind. They helped make it click for me and I've never gone back to recommending a "T" fitting or an aftermarket bias valve to anyone ever again. Those two things are:

1. Vic Elfords book on Porsche Handling. There's a section about "mid-Engine" and it touches on 914's. We get all pumped up when we tell others our cars are "mid-engine" and superior to God in handling. Vic points out that they are rather good but, the 924-944-968 package is better because, polar moment inertia can cause our little 914's to spin like a top once they get started. Having the weight at the ends (engine in front, tranny in the rear) is much more predictable and controllable. It's a good book to have and a great read when learning all that you can about your Porsche.

2. The page from the manual explaining what this little bugger really is. As stated before, it's not a proportioning valve or a bias control. It (as you rightfully stated) regulates pressure. It ONLY does this when it experiences a panic stop of 535lbs. of line pressure. It shuts the rear circuit down and slowly brings it back in to prevent the rear wheels from locking up. This keeps as much control as possible in the rear of the car in such an instance (no screaching, locked up tires back there).

Bottom line; the pressure regulator sits there and acts just like a "T" fitting, passing all the fluid you can deliver to the rear calipers unless there's an emergency where they may lock up. Then it springs into action.

The only problem is, it sucks to bleed. But it sucks even harder if you're the one bleeding.

I hope that helps someone.
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Now to try to help with your questions:

They rarely go bad HOWEVER, In an open system I see Master Cylinders go 30-40% of the time. I can only attribute that to the rubber seals seeing fresh air for the first time in a long time. My "guess" is they crack and it's new MC time. This happens too often to my clients doing a full brake job to ignore it. The reason I mention Master Cylinders is, the seals look identical and may be. I could do some research but, the Alfa guys have a kit I believe.

what are the thoughts about replacing the master cylinder with the larger one how does that effect the brake biasing?

Go for it. There's way too much internet chatter on this and the affect will be negligable. No, it doesn't affect the bias, just the overall pressure in the system.

The benifit will be a firmer pedal feel and a less expensive Master Cylinder.

The downside is, the pedal is harder to push down. It is only SLIGHT and after about 10 minutes behind the wheel you'll get use to it. This is why I say there's too much chatter going on. When it comes time, everyone should just move to a 19mm and be done with it. That's my $0.08 (adjusted for inflation and devalued for overall worth).