[BobSaabit]

In a few short miles, my brake pedal has started going close to the floor. It pumps up with no problem but seems a little spongy. Holds pressure forever once pumped. Takes 30 seconds or so to go soft with my foot off. Pedal gets a little softer when car is off but not to floor. Pumps as usual when off up to very hard. Starting car causes expected slight depression, maybe a little more than usual but not really noticeable

It’s not my abs issue - I pulled the connector and it did not help. Fluid is full. Main vacuum line looks fine (5 spd).

 

[Jeremy R.]

That sounds like your brake master cylinder to me. My parents' old Hondas used to have that issue when the master cylinder would get corroded inside. If you were sitting at a stop light with your foot on the brake, the pedal would slowly sink to the floor, but otherwise the brakes worked fine, for the most part.

 

[Boxer-Moto]

It sounds like a seal is allowing air into the system...it's not unheard of for a hydraulic seal to hold pressure, but fail when no load is on it...

I would ensure the MC is topped off (and stays topped off)...then, bleed each corner to see where the air is. One corner would point towards a likely failed caliper seal. One circuit would point towards an internal MC seal. Both circuits point towards a MC reservoir seal (or, both internal seals failing at once...less likely, but possible).

 

[Flash635]

Further to Boxer-Moto's post; one of the caliper seals might have found a scratch in the bore or a depression that allowed air in.
Or a small fault in a seal.

 

[BobSaabit]

So... I seem to have a temporary or maybe permanent reprieve: While doing my original post, I checked the exact fluid level so that I could watch that closely. Then I checked the main vacuum line with a quick exam and pushed at the connectors to make sure it was tight. I sat with my foot on the brake pedal for a couple minutes with it pumped while I idled and posted from my phone.

When I drove away, the problem seemed to have abated. I've driven a few miles, a couple stops, and it seems normal. There might a very slight sponginess to it or maybe a tiny bit of pump but it could be normal... hard to say.

Would a leaky main vac line cause this problem? I'm thinking that the only significant thing that I did was to push that line in a little at the red connector. Perhaps it was leaking and pushing it in cured the issue? I didn't really do anything else.

 

[EdT]

Wait those thirty seconds and then gently push the pedal. If it sinks way down, I'd say it's the master cylinder seals (or bore) getting worn.


P.S. I have found when I think "brakes are doing something a bit odd", and then seem to get a reprieve, that's just an illusion. If they feel funny even just once or twice, there's something wrong. A leaky line, or bad master cylinder seals, are the most obvious reasons for pedal to drop further towards the floor.
I'm not suggesting that you are about to lose all braking, and crash into a wall, but whatever the problem is, will come back.

 

[BobSaabit]

I'll probably pull the master and booster from my parts car then for insurance. It really does seem to have gone away.

Would a leaky main booster vacuum line cause this issue?

 

[BobSaabit]

Ed’s words were prophetic. It’s back.

I’m thinking I don’t have an external air leak since the problem went away temporarily before coming back. If I was taking in air it would have stayed in there and the problem would have stuck. So I’m leaning towards the “it’s the Master”. Comments?

A few questions about pulling the Master from my parts car and installing it in my working car:
1. How long a job is this, assuming my wheel bleeders come loose, etc?
2. Is there anything I should do when removing the Master to minimize bleeding issues?
3. Will I be able to bleed it old school style or do I need to buy a bleeding device if I change the Master?
4. Should I be doing the booster too at the same time? I don’t want more work, but I don’t want to waste a lot of repeat effort if odds are with the booster possibly being the issue.
5. Anything I should know to ease the pain along the way?

Can u tell I’ve never touched the Master before? LOL.

 

[BobSaabit]

... and just a quick update with some possible clues to add to the above post.

Once again I sat with my foot on the pedal ( car off) for a few minutes while I laboriously typed the previous post on my phone. Once again the problem was gone when I started the car and was fine for several miles. But, a previous shut off for a few minutes without my foot on the pedal didn’t help at all. Either posting here fixes things or the car likes it when I sit with my foot on the pedal/pumped for a few minutes

 

[EdT]

Pushing on the pedal increases the pressure in the master cylinder, which spreads the cup-shaped seals against the cylinder wall. As the seals get worn, it gets harder and harder for them to seal against the wall. I'm thinking that sitting with the brakes firmly on for a while might just reshape the cups for a while, so they continue to seal properly.


P.S. I would go for a good quality new (or rebuilt) master cylinder over a used one. Unless the maintenance on that car has been impeccable.

 

[BobSaabit]

OK boys... looks like I'm going used. Rebuilt or New is too much money for a car I don't know I'm going to keep.

Give me any tips I need for removal and easy bleeding after the install. I'm good at old-school caliper bleeding but I've never done a master replacement.

 

[jakejm79]

If you get a used one, verify the year it comes from, mid way through the 9-3 it changed and I think the bolt pattern on the booster reversed along with the orientation of the output ports to the ABS pump.

I've never done anything special regarding bleeding them, just used a pressure bleeder and ran enough fluid through till new stuff comes out of the bleeder fittings at the caliper.

 

[EdT]

It wouldn't hurt to check out "bench bleed master cylinder" on your favourite search engine.

 

[andykao]

if it helps, you can get a brake cylinder repair kit, it's pretty cheap, got mine for around 10-15 Euros, pretty sure you can find them in the US as well, has several o rings / seals (6 or 8 of them), replaced everything in my original brake cylinder after cleaning it properly, works like new.


not sure about the bleeding, my friend's a mechanic and he did all the bleeding himself (i wasn't there to check out how it's done).

 

[BobSaabit]

I pulled the previously working master from my parts car a few days ago. I didn't realize how easy that job was... I had visions of much more intricacy in the hook up.

As soon as I have a few minutes I will install it and see if the problem goes away.

 

[EdT]

I pulled the previously working master from my parts car a few days ago. I didn't realize how easy that job was... I had visions of much more intricacy in the hook up.

As soon as I have a few minutes I will install it and see if the problem goes away.

Hopefully you nave not has a few minutes yet! I hate to see someone vanish forever after saying "I'll just install this master cylinder and report right back".