ldt1

Hi all, my son just called in a panic to let me know that the brakes on my 86 951 had failed. he says there is plenty of fluid in the resevoir and that the brakes will not "pump up" with repeated depression of the pedal, and there is no visible leaking fluid around any of the tires. sounds like the master cylinder to me. is changing out the cylinder a big job? and, are the cylinders the same part on non turbo 944's as our cars. I am wondering if I will be raped if I try to pick one up locally, or would it be better to order it from paragon or one of the other mail order suppliers. Also, is there any special procedure for bleeding the brakes after installing the new master cylinder?? TIA for any helpful advice.

Charlotte944

The job is a bit messy, but not all that difficult. Have plenty of clean rags on hand to wipe up spills. Brake fluid will remove paint.

For starters you need flare nut wrenches. Normal combination wrenches will work, but yuu run the risk of rounding the nuts.

Master cylinders are available, but they can be pricey, so shop around.

Bleeding is done by starting at the passenger side rear caliper, then the driver side rear, then the passenger front, and finally with the driver front. After the brakes are bled, have your helper pump the brakes 5 or 6 times, and on the last pump hold the peddle down. While your hepler holds the peddle down, open the bleeder on one of the rear calipers, and then close the bleeder. This "resets" the brake bias valve.

One final note: The brake bias valve is the cylinder at the end of the MC. When you disconnect the brake line, hold the bias valve by putting a wrench on the flats just below the brake line fitting. When you remove the bias valve, use the flats that are close to the MC housing. DO NOT try to loosen the bias valve via the flats near the brake line, and DO NOT disassemble the bias valve.

hosrom_951

Charlotte: SO the brake bias valve comes off with the BMC as one unit?

Charlotte944

Yes. The bias valve is threaded into the rear brake circuit port, and the rear circuit hard line threads into the BBV.

hosrom_951

Charlotte: Ok, but can you explain this to me, it has been driving me crazy for a while now. Some users here on rennlist 'upgraded' or 'replaced' the stock bias valve to a 5/33? or is that the stock one?

Isn't the bias valve known as a 'pressure regulator' in the PET porsche system?

Charlotte944

I'm still in the process of learning about all of the stock bits and pieces, so I'm really not sure about upgrading the BBV.

However, you might try searching the archives.

ldt1

Well, the story changes?????? my son called again and said the brakes were woring enough to nurse the car home. I told him to park it and forget about driving anymore until I checked it out. When I fired it up the next day the brakes seemed fine to me. thought I might have detected a bit of metal to metal noise coming from the front pads but not sure. Is it humanly possible for the brakes to fail completely and then come back as if nothing was wrong. There is no reason for my kid to lie about it. there was no damage to anything that would neccessitate any fairytales

hosrom_951

Maybe it was time to replace the brake fluid. When they are 'old' they reach ta boiling point thus not work (dangerous situation when driving).

Assuming this since you started the car, and everything worked fine........

Just check the fluid's condution and level first.

Robby

Allright- this sounds like Master Cyl to me- mine went out ~1.5yrs ago- it was COLD out- I had to pump the pedal a FEW times to get pressure- then, when it warmed up (TN weather varies CONSTANTLY)- they worked fine- there are seals in the MC that start to let fluid get through or something, so, when the MC starts to go, it CAN do funny things....

I THINK I bought mine from VERTEX for ~$120- most places wanted >$200...

The STOCK bias valve is a 5/18- the 5/33 is what came on 928's & is a MUCH better choice for our cars than 5/18. Not sure WHERE it's located in 944s, but, in 951's W/ABS, it is on the passenger side- on the ABS pump or sesor or whatever- would have to ask my mech to be sure, but, he WAS under the passenger side when installing- I was up in the car ~6ft off the ground waiting to bleed... WINDWARD says that the 88 Turbo S bias valve cannot be replaced.... yeah.... whatever...

DDP

Where did you gget your 5/33 bias valve? I am looking to get one also. Thanks

Robby

Actually, I bought mine from Chris Cervelli, but, he doesn't own Technodyne anymore, so.... It's not a very difficult to find thing though. Someone here may have the part # & can tell you & you could get it through a dealer like Sunset- I'm almost 100% SURE it's the EXACT same one used on 928's (at least later years) & you SHOULD be able to just order the bias valve for an S4 or something.... I'd try Vertex & Paragon first & then go from there- I know it was <$75 & was worth it IM. Several people have mentioned going to a 5/44- there is ALSO a 5/55- the last two being 911 valves- the 5/55 would almost DEFINATELY be too much- the 5/44 has been used by some w/BR's on front, etc, but only w/ABS- I'd be afraid to use them w/OUT ABS- thing is, ABS is not supposed to be used as a bias valve, SO, I would definately not go to the 5/55. I MIGHT CONSIDER the 5/44, but..... I just stuck w/the traditional 5/33 as I knew it was quite an improvement over the 5/18 & I'm afraid to change anything TOO drastically w/this car, as something always seems to go wrong when I do....

hosrom_951

Robby: Isn't the bias valve located on the MC where the metal line goes ?

So, upgrading to a 5/33 bias valve give more stopping power (that bias valve is called pressure regulator from porsche right?)

Is it a simple remove old valve-install new valve-bleed system thing or any tricks in there?

Lindsey racing has some for sale for $54.95 which sounds like a good investment.

wh944

The p/n for the 5/33 valve is 928.355.305.02 .

Robby

hosrom-

yes- it DISTRIBUTES a little MORE of your power to the rear wheels. The rear wheels hardly do anything, proportionally, compared to the front. I used to argue the point w/people that upgrading our rear brakes would help us- in ADDITION to the fronts of course- SOME people w/NON Turbo S went to Big Reds up FRONT & then used their FRONT calipers on the REAR- some of these people installed the 5/33, from what I understand- unfortunately, I don't know anyone who has actually done this to ask about it. The Big Reds are not that much bigger than the S4 (Turbo S) calipers: ~15% more pad area, but, pistons are the same- ACTUALLY, several big brake places like KVR, &, Prospeed (I THINK that's the name), have always said that they've measured S4 caliper pistons at: 38mm & 44mm. BR's are 36mm & 44m for SURE- PET files say S4 calipers are the same(?). KVR says that S4's are better for a front engine car & that they "deflect less" under hard braking(???). Cervelli says BR's are less stressed on a 951- I'm sure you wouldn't notice a dif on a street car & I'm such a weight fanatic, that the dif in weight of the rotors would actually outweigh the benefits- BR's are cheaper, so, if I have to replace my S4's, I'll get BR's- I'd want some sort of lightweight 2-pc rotor though.....tangent... I think that regular 951 FRONT calipers are WAY too big for either BR's OR S4's & will definately increase pedal travel b/c of the extra piston size- Chris Cervelli used several dif types of MC's on his track car (to decreasse pedal travel) w/BR's front AND rear- BR rears are supposed to be same size, BUT, w/smaller pistons.

Anyway, the 5/33 bias valve is much better- the #'s have to do w/the amount of pressure the rears get before leveling off to a degree as I understand it- they do not cut the pressure OFF completely at 18psi OR 33psi, but, they make it so that a much larger percentage of brake pressure goes to the FRONTS ABOVE 18psi or 33psi... I'd kind of rather have an ADJUSTABLE bias valve, but, I'm not 100% sure it can be mounted as easily- pretty sure... Sometimes I wish I had gone adj, but, MAYBE eventually. I only went w/the 5/33 b/c it was a factory part & was a conservative upgrade... I immediately noticed a dif- it felt as if the car stayed flatter under braking- I could feel the rears playing a slghtly larger role in the process- They STILL don't do too much- I've read that the fronts do as much as >90% of the total braking under intense stops- this may be true- it would seem that the percentage would depend on how intense the force was, as weight distribution changes the harder, AND quicker, one hits the brakes- Cervelli said it only took ~30psi to lock the rears up, so, anything above 5/33 would be too much- ABS should not be used as a bias valve, but, it would HELP IF you overdid the rears. ABS was not available on your car, being an 86, so, you should either stick w/5/33, OR adjustable & BE CAREFUL w/it....

As for installation- I would have to ask my mechanic- I will do that tonight or tommorrow when I talk w/him if you'd like. I was in the car the whole time & the car was 6ft off the ground. IT DID FEEL like he pulled it pretty hard to get the old one off- the whole car shook pretty hard. It WAS underneath the passenger side for sure. But, on ABS cars, it may be dif, b/c I've been told it's on the ABS unit (whatever that means). Also, I was changing rotors, pads, lines, etc, all at the same time, so, I THINK the system was out of fluid anyway as we drained it- otherwise, you would have to lose SOME & make sure to fill it- My mech said he needed to make positively sure the air was out as he didn't THINK it would allow an air bubble to get in, but, he was not sure- that's about all I know about it, but, of all the things I've done for my car, it is one of several, like ICESHARK'S light & battery cable kit, that I would DEFINATELY do again- I've got close to $30K (counting $9K purchase price) in this car & am VERY UNHAPPY w/it overall... Hopefully someone who's done the bias valve swap here can chime in & tell you more specifically how it is done....

hosrom_951

thanks for the info Rob

Kindley let me know where it's located (and if you can take a pic that would be super )