Mismatched front calipers on 911 SC suspension swap – help identifying? Options

[bdstone914]

As I read it I think he’s saying it was leaking at the outlet port probably due to a flare nut not having been tightened fully or maybe a damaged flare.

If the master isn’t leaking out the back where the pushrod actuates the M/C it should be OK and there shouldn’t be a need to rebuild a master cylinder.

Yes, was leaking from the rear line connection. The inside bore of both fitting ports and under the rubber boot looked pretty clean. I am hopeful that it looks OK inside. If so, I'd like to replace the old rubber seals.

I can't believe the 19mm ATE MCs are so expensive now. Is it the same as Porsche Brake Master Cylinder 911-355-012-02
Is URO not worth buying?

Some members on here with Uro M/Cs have had issues from what I have read ,what the issues are I don't know ....could be QC ?? ,

It is lack of quality control. I would not use Uro master cylinder.
New rebuild kits are available from Porsche for the 19mm masters for about $100.
I have two

[Nana's_914]

Opened up the master and the bore looks OK, some debris in there, maybe from the old rubber. I think this master has been in there since 2007 or 8. Worth rebuilding?

[bdstone914]

[quote name='Nana's_914' post='3274402' date='Oct 6 2026, 05:56 PM']
Opened up the master and the bore looks OK, some debris in there, maybe from the old rubber. I think this master has been in there since 2007 or 8. Worth

Yes

[Jack Standz]

As I read it I think he’s saying it was leaking at the outlet port probably due to a flare nut not having been tightened fully or maybe a damaged flare.

If the master isn’t leaking out the back where the pushrod actuates the M/C it should be OK and there shouldn’t be a need to rebuild a master cylinder.

Yes, was leaking from the rear line connection. The inside bore of both fitting ports and under the rubber boot looked pretty clean. I am hopeful that it looks OK inside. If so, I'd like to replace the old rubber seals.

I can't believe the 19mm ATE MCs are so expensive now. Is it the same as Porsche Brake Master Cylinder 911-355-012-02
Is URO not worth buying?

Here's another way to go:

http://www.914world.com/bbs2/index.php?showtopic=312451

Currently, $28 to $72 for a new 20mm master cylinder from rock auto.

https://www.rockauto.com/?carcode=1281866&parttype=1836
.

[Superhawk996]

Be very careful you understand what you’re doing when randomly changing master cylinder sizes.

The stock master is 17mm.

A 19mm will increase pedal force required to stop by about 25%. Although this is a common swap for 914s due to the limited availability of of the stock 17mm master cylinder, it comes with the pedal force change. Some people like the increased force and reduced pedal travel, others don’t care for it. Lots of people have become inordinately used to power assisted brakes and going to a unboosted brake system already seems like it may take more force to stop than they are used to.

A 20.6 mm (13/16” bore) will increase pedal force by about 47% vs the 17mm stock master cylinder. I would seriously caution you on doing this on a manual brake system that is without a brake booster to provide some pedal assist.

The brake pedal force change is not linear to master cylinder bore diameter change.

[Jack Standz]

Be very careful you understand what you’re doing when randomly changing master cylinder sizes.

The stock master is 17mm.

A 19mm will increase pedal force required to stop by about 25%. Although this is a common swap for 914s due to the limited availability of of the stock 17mm master cylinder, it comes with the pedal force change. Some people like the increased force and reduced pedal travel, others don’t care for it. Lots of people have become inordinately used to power assisted brakes and going to a unboosted brake system already seems like it may take more force to stop than they are used to.

A 20.6 mm (13/16” bore) will increase pedal force by about 47% vs the 17mm stock master cylinder. I would seriously caution you on doing this on a manual brake system that is without a brake booster to provide some pedal assist.

The brake pedal force change is not linear to master cylinder bore diameter change.

Yes, agreed. As the first link states "As with any brake work. If you don't know what the hell you're doing, don't do this."

As for the rest of what you said:

Add a brake booster if you want to (and know what you're doing). No disagreement with that.

Many of us have modified our 914s. Things like A calipers, M calipers, S calipers come to mind. These calipers have larger diameter pistons and require greater volumes of fluid (compare their pistons to 17mm or 19mm master cylinders before you modify things). For example, we are running S calipers and M calipers on a '74. But, we also have a NOS 17mm that's earmarked for the '72 project when we get to the point of restoring the brake system (yes, a system - a system that needs to work right together). However, we have no heartburn putting g a 19mm master cylinder in the '72 or a 20mm in the '74, if necessary. YMMV

[Superhawk996]

All fair points and I should have noted the 17mm comparison is to the 914 and really what OP is running is 911 fronts (?? On rear) and the 19 is the baseline for the 911.

Just be careful out there.

[Jack Standz]

And I think the 911S used a 20mm master cylinder with the "S" front calipers anyway. So, our 19mm master cylinder with S front calipers it's actually undersized compared to a stock 911S (not accounting for the weight difference between the 911 and 914).

[Nana's_914]

Thanks everyone for the constructive feedback.

System plan so far:

Front: '78 911 SC struts with a matched pair of A calipers (48mm pistons), Sebro cross-drilled 20mm vented rotors, and soft lines that transition from 16mm on the 914 chassis tabs to 14mm on the SC strut tabs. What's the trick to adapting to the SC strut tab? 16mm-to-14mm lines, drilling the tab, or other?

Rear: The car has a PMB vented 914-6/GT rear kit, but the calipers need a refresh, and I suspect they're spaced stock calipers rather than true GT units. If they're stock 33mm, I'll think about upgrading to the 38mm GT calipers to restore the factory 914-6/911 balance with 48mm fronts.

Regulator: Rear pressure regulator calibrated by PMB for this caliper combination.

Master: Rebuild the 19mm with a Porsche kit, which matches 914-6 spec with 48mm fronts and 38mm rears, then evaluate pedal travel vs. effort. A 20.6mm stays in my back pocket if the rebuild fails or the pedal feels long.

Lines: New hard lines and braided soft lines throughout.

I'd like to verify the rear caliper bore without taking them apart. The piston face step measures ~29mm (the boot hides the rest), and the pad backing plate measures ~56mm x ~44mm, not including the pin ears.