924Board.org

[coup85]

Hi everyone,

I’ve been working hard to sort out a braking issue on my 1979 Porsche 924 2.0 NA and could use some advice from the community. I’ve done everything by the book (twice), but I still have a soft brake pedal only when the engine is running.

Situation so far:

I’ve replaced the brake master cylinder three times:

Original used unit (pulled from the car)
Refurbished unit from a breaker
Brand-new aftermarket master cylinder, currently installed

I’ve done multiple full brake bleeds using a pressure bleeder (Motive-style):

First round was a bit messy (reservoir overflow, loose tap on the bleeder cap)
Second round went cleanly: pressure held steady (~1 bar), no leaks, no air drawn in
Cylinders and all four wheels bled thoroughly, twice

All fittings hold pressure, including the bleeder cap. I left the system under pressure for minutes during the process and saw almost no drop (~2–3 psi loss total).

I’ve confirmed:

No visible leaks
No fluid loss
No bubbles in the lines during the second purge
Rear brake shoes are adjusted (I hear them contact when brake is pressed with the handbrake off)
Rear drums are not seized and move freely

Current symptoms:

With engine off:
Pedal is firm
Holds pressure, does not sink over time

With engine running (servo active):

Pedal becomes soft
Can be pushed down almost to the floor
Car does brake, but the travel feels excessive

Additional context:

When I press the brake with the handbrake off (engine off), I hear the rear shoes shift and contact the drum—suggesting some small play remains.
I’m using DOT 4 and replaced all bleeder screws.
No flex lines have been replaced yet, but none appear damaged externally.
I flushed all fluid after realizing the reservoir had been left empty during a previous attempt.

What I’m considering:

Could the brake booster (servo) be amplifying travel due to internal contamination from an earlier master cylinder leak? I did find fluid in the booster early on and cleaned it out as best I could.
Could the new master cylinder have an internal seal issue, even if it’s new?
Could the rear shoes still be too far out, even though I feel some drag when rotating the drum?

Any thoughts or similar experiences would be hugely appreciated. I’ve put a lot of hours into this…
Thanks in advance!
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924 - 79'

[morghen]

Mine was doing something similar.
The booster was bad, had fluid inside.
You should replace the flexible lines as well, can be a failure making a bubble somewhere in the rubber and it only gets the force to overcome the rubber wall when servo is active.
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Supercharger and EFI kits
https://www.the924.com

[coup85]

Thanks! It’s incredibly helpful to hear that someone experienced the exact same issue, I was becoming crazy…

I have a couple of follow-up questions if you don’t mind:

Is there any way to visually inspect which brake hose might be internally failing? All mine look okay from the outside — no cracks or bulges — but I understand the problem could be internal and only show under pressure. I’d like to confirm before ordering new ones.

Has anyone here ever opened or rebuilt a 924 brake booster? I cleaned it as best I could but not sure if the diaphragm might have been damaged. Is the booster serviceable at all, or are they sealed permanently?

Can I use a brake booster from any similar models? (like early 944s, VW/Audi equivalents, etc.). Mine is proving hard to source at the moment, so I’m exploring cross-compatibility options in case I can’t find an exact match.

Thanks again

PS: While is true I found some fluid on the booster, it wasn't much.
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924 - 79'

[morghen]

There should not be any fluid in the booster.
It may be compromised. You can use an early 944 booster.

Flexible lines are consumables, you should not wait until they fail to change them. Just change them and dont play around with that. They can even fail internally and create some cavity in between the layers.
_________________
Supercharger and EFI kits
https://www.the924.com

[coup85]

Thanks, that sounds like solid advice.

I’m planning to test each flexible line by clamping them individually — just to see if isolating one of them restores pedal feel, or if the issue points directly to a compromised booster.
That way I’ll know whether to replace just the lines or both.

Does that sound like a reasonable approach or I'm missing something?
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924 - 79'

[Raize]

Sounds like the brake servo is just doing its job.

Unless you're pushing on the brake pedal with 100kg+ of course it's going to be stiff. The force generated by the servo is enormous.

There's no way a ruptured diaphgram can make pedal travel longer, you would just lose your power brakes.

A failed seal at the back of the master cylinder would give you a longer pedal travel but you'd also be losing HUGE amounts of brake fluid.

[coup85]

Thanks for the thoughtful reply — I really appreciate the input.

You’re absolutely right that the servo is supposed to apply a lot of force, and a fully ruptured diaphragm would just result in a loss of assist, not a longer pedal. My situation is a bit strange though: with the engine off, the pedal is firm and holds; but as soon as I start the engine, the pedal immediately sinks almost to the floor. It still brakes, but the travel feels excessive — more than what I’d expect from just normal servo assistance.

I’ve already replaced the master cylinder three times (including a brand new one), and I’m not losing any fluid, so I’m fairly confident the MC isn’t the culprit here.

As for the flexible lines, they’re “relatively new” but I don’t know the brand or history, so I’m going to clamp them one by one this weekend and see if that changes the pedal feel. If it does, that’ll help narrow it down without replacing parts blindly.

Your point about the front calipers and rear drums makes a lot of sense too. My mechanic previously removed the front pads to see if they had to be replaced, so I want to double-check their seating and hardware. The rear drums are adjusted to slight contact, but I know it takes some time for the shoes to fully conform to the drum radius.

Thanks again — this has all been super helpful in refining where to look next.
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924 - 79'

[Raize]

Hope you get it sorted, curious to know the result of clamping the lines off, that’s a very good test.

I will be installing some four-pot front brembos and modern sliding caliper rear brakes in the next two weeks so will be interesting to see if I have a better pedal. It may actually be worse because of the increased piston diameters but either way I’ll be doing a write-up.

[coup85]

Quick update: Someone warned me about the possible misadjustment of the pedal on its union with the booster inside the cabin. I'll be checking that first as it could explain my synthomps. Sadly that part of the car is pretty rusty so is going to take some time to make the test.
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924 - 79'

[Raize]

I suppose the wildcard here is that the pedal linkage free play needs to be set with the engine running. That’s not in the manual but it would make sense.

[coup85]

Found a major issue — the rear right brake line had a broken fitting where the hard line disconnects from the flare inside the nut. No visible leaks, but definitely a pressure loss point. After capping both rear flex lines, the pedal became much firmer with a significantly reduced travel. I’ll replace the rear lines next and retest the full system.

I'll update with any news.
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924 - 79'

[Beartooth]

Sounds good; hopefully new lines fix your problem. I had a mess of a brake system on my car: one or both wheel cylinders were leaky and as a result, the reservoir was empty and some damage occurred to the master cylinder while sitting for around a decade. I replaced the master cylinder and all four flex lines, rebuilt the calipers, and replaced both wheel cylinders. Then I blew out the brake lines one by one with compressed air (disconnect the wheel cylinder or caliper - one at a time - put a rubber hose on it, and run it into a jar to catch the mess). The result was brakes that worked perfectly from day one. Obviously, you may well have a problem I didn't have, but I think that approach eliminates most potential problems. One other thought: have you fully inspected the brake lines? If one is rusting through somewhere you're not seeing it, that could explain the problem. If the problem is caused by a leak, it should be visible (unless it's leaking into the booster), and you'd be losing fluid... Ultimately, I think it's best on a compromised or unknown system to replace all the rubber pieces that see brake fluid; definitely flex lines at a minimum (they should probably be replaced periodically, maybe every ten years).
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1980 931 diamond in the rough

[coup85]

Thanks! Yes, I agree with your approach — it’s the most thorough and reliable way to bring the system back to life. In my case, though, I’m working with a limited budget and trying to pinpoint the failure without replacing everything at once.

Ideally I’d replace all flex lines and wheel cylinders eventually, but for now I’m focusing on confirming whether this one failure was the main cause.
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924 - 79'

[coup85]

Update:
After discovering a damaged hard line at the rear, I decided to investigate further. One of the T-junctions in the rear brake lines started leaking as soon as I moved it, and broke shortly after with minimal force — clearly too corroded to trust. Given the condition of the fittings and lines, I’ve decided to rebuild the entire brake hard line system from scratch.

This will go along with the new soft lines I already had planned to install. It’s going to be a big job, but I’d rather do it right once than chase issues one by one.

Thanks again for all the guidance so far — I’ll keep updating as I make progress!
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924 - 79'

[Raize]

Unfortunate, but better to find out now than when you are driving.

Make sure you get genuine cupronickel pipe - you can do a magnet test, it should be nonmagnetic. The cheap fake ones are copper plated steel.

You can also use a magnet to find out which hard lines in your 924 have already been replaced with cupronickel. Might save you replacing a few of them unnecessarily.

The main thing you have to watch out for with the cupronickel lines is if they twist along with the fitting when you try to remove them. If that happens, they need replaced unless you have enough slack to cut out the fatigued section. If you SEE it trying to happen, just cut the line and you can save it with a new fitting and flare.