Brake Question

[Salonika]

And what about the front brake lines?

View attachment 717631

The pedal is so bad I can’t see how flushing the fronts will fix it……no way has that much air just gotten in there without a fluid leak showing up somewhere. I’m on the original factory master cylinder, for a 1998, so it’s due to be replaced anyway.

 

[Peter_n_Margaret]

Master cylinder plunger leaking?
Cheers,
Peter
OKA196 motorhome

 

[Salonika]

No fluid leaks at all, anywhere. And prior to symptoms getting really bad, no recent brake service either for over a year.

 

[Salonika]

Look at the picture. The system consists of two parts, each of which connects one rear and one front wheel. After removing the air from the rear wheel, it is necessary to remove the air from the front wheel, which is connected to this rear wheel. If this is not done, then removing air only from the rear wheel will not help to remove air from the corresponding front wheel. The order in which you need to remove air from the system is also shown in the figure: first 1 and so on.

I know what a pedal feels like with air in it……that’s not what this is to me. The pedal literally requires two pumps to build pressure, first pump goes to the floor, and I mean to the floor against the hard stop. The second pump then acts 100% normal braking. There is no way that I got air in the system to the amount that would explain this. A few small bubbles, if present, make a pedal feel “soft” but not like this,,,,,,,this isn’t a soft pedal even.

 

[Peter_n_Margaret]

Master cylinder plunger leaking?

No fluid leaks at all, anywhere. And prior to symptoms getting really bad, no recent brake service either for over a year.

A leaking plunger in a master cylinder does not leak brake fluid. It loses pressure.
Cheers,
Peter
OKA196 motorhome

 

[Salonika]

The weirdest part about my problem is that when I do the pump & hold test, the pedal doesn’t sink (on the second pump obviously).
Im still going to bleed the front calipers too so I can rule out air completely and complete my fluid flush. If that doesn’t fix it, I’m replacing the master cylinder. It is 24 years old.

 

[Salonika]

I had a similar problem. But not quite the same. The brake pedal was very soft when first pressed, but it didn't touch the floor. If you click on it a second time, everything will work as it should. I have completely removed the air from the system. It didn't help. I replaced the master cylinder. This did not solve the problem. I checked the brake calipers and discs. I got a result close to perfect. I removed the brake force regulator between the axles. The brakes continued to work poorly. And I replaced the brake hoses. The problem has disappeared! But I must repeat: the pedal did not touch the floor when first pressed. The pedal was just very "soft".

My hoses are all in spec on the maintenance replacement calendar (less than 10yrs old). Yes as hoses age they become softer and some of your brake pressure potential is lost due to the hoses actually expanding a bit. But no way would that explain my behavior, as you said, your pedal wasn’t hitting the hard stop like mine. I’m really leaning towards the MC mostly because of age and process of elimination. Also, I have noticed clouds of black streaks coming up out of the supply holes to the pistons…..and my reservoir fluid has gone black very easily in the past few years…..I even replaced the cap thinking the rubber was degrading…..I think the piston seals have worn out and finally are deteriorating very fast. At least it isn’t a terrible job to do on the Montero. Looks pretty easy to get at. Worst part of job is re-bleeding after it’s done. Also hoping not to get air trapped in the ABS.

 

[Salonika]

UPDATE: while trying to flush the front circuits I broke off the bleeder screw for the front right. When I took off the caliper for replacement this is what it looked like. It was not leaking. Both pistons looked this way, and this is what it looked like after I slid it off the rotor with brake line disconnected so this is how it looked driving. I think my spongy brake pedal was from this seal bulging in the first pedal pump. How did this not blow out? I replaced this caliper and kept the pads. Flushed and bled front circuits. Brakes are back to normal, so it wasn’t the Master Cylinder after all. I can’t quite make sense of it, but it is what it is.