Master Cylinder Issues-Help Please

[Uwon]

Hey Guys,
Background: RST build under construction Evolution of a Carrera RST. '86 master cylinder and brake lines, 930 calipers, etc. into a '78 SC chassis.
So, I'm charging the brake lines with my trusty Motive bottle/pump. I get nice clear brake fluid to all corners, a couple of leaks which I fixed. I try the brake pedal-NO pedal pressure no matter how long I pump whether or not the Motive is under pressure.
Pedal travels only 3.75" when measured against the bulkhead which seams a bit short.
The top of the Reverse Lever (911-355-083-03) only travels .70" (17.80mm). See pic below where Igot this measurement.



I have never disassembled a master cylinder before so I don't really know how far it should travel. It just seams odd to me that the travel is so short which may explain why I cannot get pressure.
So, I sure would like to hear from the braintrust.
Thanks in advance.
Johan

[john walker's workshop]

Czech republic.

[javadog]

If you have 930 calipers on both ends and are using a Carrera M/C, you are using the wrong M/C.

If it is new, see John's post above.

JR

[john walker's workshop]

Yeah, a 930 cylinder is 930 355 011 03.

[DRACO A5OG]

Not for nothing, but put your old Carrera calipers on to confirm the MC is working properly

[Bill Verburg]

Originally Posted by Uwon

Hey Guys,
Background: RST build under construction Evolution of a Carrera RST. '86 master cylinder and brake lines, 930 calipers, etc. into a '78 SC chassis.
So, I'm charging the brake lines with my trusty Motive bottle/pump. I get nice clear brake fluid to all corners, a couple of leaks which I fixed. I try the brake pedal-NO pedal pressure no matter how long I pump whether or not the Motive is under pressure.
Pedal travels only 3.75" when measured against the bulkhead which seams a bit short.
The top of the Reverse Lever (911-355-083-03) only travels .70" (17.80mm). See pic below where Igot this measurement.



I have never disassembled a master cylinder before so I don't really know how far it should travel. It just seams odd to me that the travel is so short which may explain why I cannot get pressure.
So, I sure would like to hear from the braintrust.
Thanks in advance.
Johan

The spec for the 930 m/c stroke is 18mm

[Uwon]

Originally Posted by Bill Verburg

The spec for the 930 m/c stroke is 18mm

Thanks Bill. At least I'm within range.

Let me explain further. The master cylinder assembly, lines and brake calipers came off my donor crashed car which was a setup that I used for over 10 years with great success at the track. I think that I did upgrade the master cylinder itself to the 930 as it is identified with a number 23 on the right side. See pic



Can anybody verify that the Master cylinder is indeed from a 930?
The bearing bracket is 91135538201 which is common to the Carrera and the 930, from what I understand looking at the PET.
The master cylinder was only recently removed from the crashed car and it still had fluid in it so its not as if it dried out, I don't think. So, why still no pressure?
Johan

[Bill Verburg]

Originally Posted by Uwon

Thanks Bill. At least I'm within range.

Let me explain further. The master cylinder assembly, lines and brake calipers came off my donor crashed car which was a setup that I used for over 10 years with great success at the track. I think that I did upgrade the master cylinder itself to the 930 as it is identified with a number 23 on the right side. See pic



Can anybody verify that the Master cylinder is indeed from a 930?
The bearing bracket is 91135538201 which is common to the Carrera and the 930, from what I understand looking at the PET.
The master cylinder was only recently removed from the crashed car and it still had fluid in it so its not as if it dried out, I don't think. So, why still no pressure?
Johan

The m/c bore is cast into the side so that appears to be a 930 m/c, a

[ClickClickBoom]

My bet is crud keeping the seal on the m/c piston from working. Pull the m/c out and have a look, in the bore and around the seals, the breaking of the lines and moving the components exposed to the environment allows plenty of opportunity for the crud to attack. The other possibility is a proportioning valve failure.
Brakes are relatively simple systems, but it doesn't take much to make them go jiggy.

[Uwon]

Originally Posted by ClickClickBoom

My bet is crud keeping the seal on the m/c piston from working. Pull the m/c out and have a look, in the bore and around the seals, the breaking of the lines and moving the components exposed to the environment allows plenty of opportunity for the crud to attack. The other possibility is a proportioning valve failure.
Brakes are relatively simple systems, but it doesn't take much to make them go jiggy.

I am planning to pull out the whole unit tomorrow. However, a couple of your comments I intrigue me and raises some further questions:
1. Isn't there a spring in the master cylinder that pushes the piston back to the forward position?
2. My brake pedal returns home by itself. If the m/c piston is stuck, then what else would push the pedal home? Something in the booster?
3. The PET is missing a link between the booster and the pivot lever. Can it have fallen out?
Johan

[ClickClickBoom]

Not having been in a Porsche m/c, I will suppose they have a piston spring to return it into battery, and the pedal cluster might have a spring as well. A missing or wrong linkage length could be a factor as well. If the piston isn't returning to correct battery, the reservoir will not be able to fill the m/c for pressurization.

[Uwon]

Since the fluid gets to all four corners whether or not the pedal is depressed, I suspect that the piston does not move from the "at rest" position. So I think that the issue is, as you have suggested, the connection between the pivot lever and the piston. Unfortunately the PET is not clear as to the bits in between. A search in various forums did not reveal anything either. I'll take some pics today and post.
Johan

[Uwon]

Joke is on me! Really pi**off actually at myself.
I did not check out the assembly carefully enough prior to installing in the RST build. The whole assembly was damaged (bent) from the donor accident so the pin from the booster got bent along with the piston resulting in a master leak into the booster housing.







Went ahead and ordered a new 930 master cylinder. Now looking for a used booster.
Thanks everyone for your interest suggestions.
Cheers,
Johan

[DRACO A5OG]

Ah Crap. Sorry to see that Brother!

[javadog]

I have a used 930 booster for sale.

JR

[Jonny042]

I'm in ON too and could ship a booster from a 1978 SC to you this afternoon. Overnight via Canpost would probably have it to you mid-week

[88911coupe]

Slightly OT, but what particular orange is that paint? Really great color. Other than Fishcops car (blue) my favorite classic Porsche colors are the oranges but I get confused over the actual names. Is that Signal Orange?

[Uwon]

Originally Posted by javadog

I have a used 930 booster for sale.

JR

Read your post. Nice Unit but I think that I need the larger later model for my build.
Cheers,
Johan

[javadog]

That's up to you. With a car that light and turbo brakes, the large booster is going to give you the pedal effort of a Buick.

JR

[Uwon]

JR, interesting comment. The '86 donor which was essentially the same setup but probably 200 lbs heavier had no problem stopping. I carried over the adjustable proportioning valve for the rear which worked well for me at the track in both dry and wet. Notwithstanding, I am warming up to the idea of adding another bit out of an old 930.
Johan