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PopUpProject
Joined: 06 Aug 2025
Posts: 111
Location: NE Ohio
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 Posted: Fri Dec 19, 2025 11:38 pm Post subject: What doesn't work from this brake MC rebuild kit? |
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Hi guys,
After the rear drums rebuild and bleeding the brakes I'm back and continuing my brake saga. Upon bleeding, my passenger front and driver rear bleed fine, but the driver front and passenger rear bleed pitifully and we put new bleeders in all around. Therefore we're thinking the MC is bad and needs rebuilt so I've been hunting for either a rebuilt MC or a kit, and from a previous post https://www.924board.org/viewtopic.php?t=46133 there's a different kit that doesn't include the plungers and would have to be imported from across the pond. I also wasn't able to confirm if that kit works or not as its an older post. But for a $15 kit, I don't want to pay outlandish import fees (literally quadruple the price for importing if I want it before the middle of January) if it's not even complete and may not work.
I know on the parts page it is said that mc repair kit 928-355-913-02-OEM (Pelican PN for 928-355-913-00) doesn't have enough in it to fully rebuild the 477 611 017 A brake MC. However, what exactly doesn't work on it? Is it the bore size of the plunger things, do the seals not seal properly? I called Porsche and according to the dude I talked to it seems like it should work just fine, but they couldn't tell me 100% if it would. I'd be willing to pay for the oem Porsche rebuild kit if it actually works and seems to have all the bits required
Yes I'm using very technical terms, but I'm ready to pull my hair out working on these brakes as I know its only gunna go downhill from here XD
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Poppy the '82 924 Resurrection in Progress
Last edited by PopUpProject on Sat Dec 20, 2025 8:56 am; edited 1 time in total |
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924RACR
Joined: 29 Jul 2001
Posts: 9060
Location: Royal Oak, MI, USA
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| Posted: Sat Dec 20, 2025 12:28 am Post subject: |
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That's certainly a very complete kit, with the pistons and all... I would be most concerned about the main seals in the bore being what need replacing... and so then it's a question of how would a 928 be likely to still have the same bore sizes for a 4-bolt 924?
I'm definitely skeptical, and that's one hell of a lot of money to gamble.
Before you go that far, just a couple of quick sense checks.
Have you replaced the hoses already? We've definitely seen cases where old rubber hoses collapse and restrict flow. You might even consider disconnecting one of the rear brake lines right at the master and see what the flow is like there, before it enters the hard lines, just to isolate the master as a source of the problem vs. the rest of the car. Maybe use a short bit of spare brake line in the port to direct the flow into a catch can while a helper pumps the brake...
If none of the above, are you sure you're getting enough pressure and stroke at the pedal to activate the rears? Sometimes, with such a big hydraulic difference on a brake system, the rears end up taking a lot more activation at the pedal to get much out of them. Of course, testing the flow right up at the master would help clarify that too...
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
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PopUpProject
Joined: 06 Aug 2025
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| Posted: Sat Dec 20, 2025 8:42 am Post subject: |
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| 924RACR wrote: | That's certainly a very complete kit, with the pistons and all... I would be most concerned about the main seals in the bore being what need replacing... and so then it's a question of how would a 928 be likely to still have the same bore sizes for a 4-bolt 924?
I'm definitely skeptical, and that's one hell of a lot of money to gamble.
Before you go that far, just a couple of quick sense checks.
Have you replaced the hoses already? We've definitely seen cases where old rubber hoses collapse and restrict flow. You might even consider disconnecting one of the rear brake lines right at the master and see what the flow is like there, before it enters the hard lines, just to isolate the master as a source of the problem vs. the rest of the car. Maybe use a short bit of spare brake line in the port to direct the flow into a catch can while a helper pumps the brake...
If none of the above, are you sure you're getting enough pressure and stroke at the pedal to activate the rears? Sometimes, with such a big hydraulic difference on a brake system, the rears end up taking a lot more activation at the pedal to get much out of them. Of course, testing the flow right up at the master would help clarify that too... |
Thanks Vaughan! Yeah........... I don't want to drop the money if I don't have to but its also cheaper than a new MC.
We have not replaced the hoses yet as they look new-ish. I can tell they were probably on the cheaper side based on how the metal connectors look now, but the rubber seems to be okay and pretty new compared to the rest of the rubber on Poppy. But isolating the lines from the MC seems to be a good idea, I just gotta pray they don't crumple when we take them off as they look super questionable but in their current state have no leaks.
As far as pedal pressure goes, before the rear brake rebuild, there was absolutely no brake pedal feeling. The pedal would go straight to the floor and turtle crawl its way back up. The tear down confirmed a leaky cylinder that had been active for a long time based on the amount of crud built up and having basically no fluid left in the reservoir. During bleeding I got thrown up in air jail and I was sticking my foot fully to the floor decently hard and fast, then we tried slow and steady, and we also tried just like pumping the brake pedal and we still got piddly streams out of those two. I did regain some pedal feeling (i.e. more than nothing and it'll rebound back up) but its still pretty mushy and will occasionally go straight to the floor.
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Poppy the '82 924 Resurrection in Progress |
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Raize
Joined: 18 Sep 2013
Posts: 445
Location: Scotland
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| Posted: Sat Dec 20, 2025 8:10 pm Post subject: |
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What do you mean "bleed pitifully"? If the pedal is hard and the brakes are coming on, it's fine, the rate of fluid stream you get out at the bleeder is irrelevant. It just needs to not have air bubbles in it.
Brake systems do not need high flow, they are hydraulic. Pascals law. |
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924RACR
Joined: 29 Jul 2001
Posts: 9060
Location: Royal Oak, MI, USA
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| Posted: Sat Dec 20, 2025 10:35 pm Post subject: |
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Best protection against damaging the lines on removal is using a proper hydraulic line wrench, the type that wraps around the whole nut bar one, and of course letting a little penetrating oil soak in first. But the wrench is key.
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
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PopUpProject
Joined: 06 Aug 2025
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Location: NE Ohio
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| Posted: Sat Dec 20, 2025 10:48 pm Post subject: |
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| Raize wrote: | What do you mean "bleed pitifully"? If the pedal is hard and the brakes are coming on, it's fine, the rate of fluid stream you get out at the bleeder is irrelevant. It just needs to not have air bubbles in it.
Brake systems do not need high flow, they are hydraulic. Pascals law. |
The pf and dr would have consistent streams that hit into the wheel well but the other two would just barely have fluid come out, best as we could tell all of the air is out of the lines as at towards the beginning of bleeding we got a lot of bubbles and air pockets. I also wouldn't call the pedal feeling hard, its mushy and spongy at best.
I will have to look for that wrench Vaughan, hopefully my buddy has one at the shop and if not, I'll be picking one up
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924RACR
Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA
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| Posted: Sun Dec 21, 2025 2:07 am Post subject: |
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Yeah, you should absolutely be able to get a stream that'll hit the wheel well out of any corner. Yes, it functions on pressure, but volume and thus flow IS needed, and if you can't generate flow without restriction, yes something is wrong.
On the plus side, if you do need to buy the line wrenches - you don't need to go all out on expensive, the parts store brand stuff like Performance Tool, Harbor Freight, etc are sufficient, since it's just about getting the nuts to crack the first time, and not torquing the line in the process (which is what the penetrating oil is for). So that can keep the cost down. Just make sure you have a tight fit before turning - get all the corrosion off the nut and the wrench should be a snug fit on the line nut. They'll be probably 12mm at the master, IIRC - possibly 11mm, but definitely not 13.
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Vaughan Scott
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PopUpProject
Joined: 06 Aug 2025
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| Posted: Sun Dec 21, 2025 3:33 am Post subject: |
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| 924RACR wrote: | | Yeah, you should absolutely be able to get a stream that'll hit the wheel well out of any corner. Yes, it functions on pressure, but volume and thus flow IS needed, and if you can't generate flow without restriction, yes something is wrong. |
Sweet, I was hoping it wouldn't be a specialty porsche wrench. If we're not getting flow at the MC what could it be? Just one of the internal seals?
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924RACR
Joined: 29 Jul 2001
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| Posted: Sun Dec 21, 2025 3:42 am Post subject: |
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Presumably, yes.
Have to say, though, I'd generally expect the failure mode of the master would be leaking past the seals such that pressure could be made, but not maintained... which would be more like initial pedal height slowly dropping away.
At least I think that's what I remember from when the one in my Turbo died...
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Vaughan Scott
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PopUpProject
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| Posted: Sun Dec 21, 2025 3:57 am Post subject: |
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| 924RACR wrote: | | I'd generally expect the failure mode of the master would be leaking past the seals such that pressure could be made, but not maintained... which would be more like initial pedal height slowly dropping away. |
When we first picked the car up there was old fluid all over the front of the brake booster but we couldn't find any fluid coming out from the general area, I'm not sure if that means anything or if that happened when someone got messy trying to refill the reservoir. If I have someone keep eyes while we bleed the brakes with the lines still connected, would it be possible to see if we get seepage from the mc or the booster?
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Raize
Joined: 18 Sep 2013
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Location: Scotland
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| Posted: Sun Dec 21, 2025 4:05 am Post subject: |
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| 924RACR wrote: |
Have to say, though, I'd generally expect the failure mode of the master would be leaking past the seals such that pressure could be made, but not maintained... which would be more like initial pedal height slowly dropping away. |
That sounds about right.
We can rule out the external seals or there would be brake fluid everywhere.
If an internal seal has failed then you would definitely find that if you closed all the bleeder screws the pedal would either go straight to the floor (if it had a major failure) or it would drop to the floor over time as the fluid slowly leaks past the seal back to the reservoir.
So it would actually be a good diagnostic to put all four bleeder screws in tight and just see if you are getting that behaviour before tearing into the master cylinder.
To be honest I do have trouble getting the brakes bled properly on my car, it's never felt quite right but it works so I just live with it. Pretty sure there is a stubborn air bubble in the lines at a high point or something that's impossible to get out. |
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PopUpProject
Joined: 06 Aug 2025
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Location: NE Ohio
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| Posted: Sun Dec 21, 2025 4:14 am Post subject: |
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| Raize wrote: |
If an internal seal has failed then you would definitely find that if you closed all the bleeder screws the pedal would either go straight to the floor (if it had a major failure) or it would drop to the floor over time as the fluid slowly leaks past the seal back to the reservoir.
So it would actually be a good diagnostic to put all four bleeder screws in tight and just see if you are getting that behaviour before tearing into the master cylinder.
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Alrighty, we'll have to try that before we crack the lines off of the mc. This might be a stupid question, but when you say pedal will drop down to the floor, is that when I'm pushing on it? Or is that like it'll find the floor on its own? And if its the it'll fall down on its own, how much time would yall expect for it to take for the pedal to do so?
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Poppy the '82 924 Resurrection in Progress |
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Raize
Joined: 18 Sep 2013
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| Posted: Sun Dec 21, 2025 6:22 am Post subject: |
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Just when you're pushing on it.
Keep hard pressure on it for a minute or two. If there's a nick on the master cyclinder bore, or a slightly torn seal, the fluid will slowly get pushed past that and you'll eventually push the pedal right down to the carpet.
But if it's just some air you're compressing, it'll not move any further. |
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PopUpProject
Joined: 06 Aug 2025
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| Posted: Sun Dec 21, 2025 6:30 am Post subject: |
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| Raize wrote: | Just when you're pushing on it.
Keep hard pressure on it for a minute or two. If there's a nick on the master cyclinder bore, or a slightly torn seal, the fluid will slowly get pushed past that and you'll eventually push the pedal right down to the carpet.
But if it's just some air you're compressing, it'll not move any further. |
Gotcha, thank you!!! I'll have to do that the next time I go over to the shop. I'll report back with my findings hopefully here in the near future
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Poppy the '82 924 Resurrection in Progress |
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924RACR
Joined: 29 Jul 2001
Posts: 9060
Location: Royal Oak, MI, USA
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| Posted: Sun Dec 21, 2025 7:35 am Post subject: |
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I will mention, while I have access to the very best and most complex bleeder systems known to man... on ALL our cars, ESPECIALLY the racecars, we strictly rely on the oldest mode of steering wheel spacer (aka driver) pushing on the pedal to make the pressure.
Vacuum or pressure at the master reservoir all can get you quickly in the zone, but we never find as good a pedal on our cars as just pushing on the pedal. And make sure, for that final squirt, that you close the bleeder off while the pedal's still dropping/caliper is under pressure...
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype |
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