I'll give the full back story to this in order to try avoid wasting anyone's time with follow up questions.

1998 Discovery 1, with ABS.
The initiation of the issue was after I installed new rear rotors and pads. Once everything was back together I couldn't get the calipers to bleed. I tried both the two man bleed method and a MityVac hand pump, both to know success. I ended up buying a pressure bleeder so that I could stop calling my wife to the garage to push on the brake pedal. However, despite that building pressure, no fluid came through to the calipers, even the front ones. I finally tracked the issue to the master cylinder. The pedal bleeding had pushed old corrosion to the point of blocking the outlet and no fluid was getting out of the master cylinder.

So, I replaced the master cylinder with one from Atlantic British. From there the system bled normally (pressure bleeder) and I was able to get pressure back. However, upon driving the car the first time, the brakes became spongy. I checked for leaks and found oil on the passenger rear rotor. I tracked that down to a blown stub axle seal, but replaced the caliper pistons and seals for good measure, along with new pads. Then I bled the system again for good measure. Again, when I drove the car the brakes became spongy again. I drove it this way for a few months, just occasionally for local errands, as I didn't have time to further troubleshoot. Although the brakes didn't inspire confidence they were still functional.

Then a few weeks ago, I found a leak at the driver's front caliper. This was clearly from caliper the pistons and seals and so I replaced them, along with new pads. Again, I bled the entire system at each caliper. The pedal felt solid as a rock. However, as soon as I drove, it became spongy again. So, I spent today doing some additional testing.

1) Inspected for leaks at all hoses and calipers. Found none and fluid level in reservoir is not changing.
2) Inspected seals where the reservoir meets the master cylinder (even though they're less than a year old). No leaks
3) Sprayed the reservoir cap with soapy water to check for air leaks there, but nothing.
4) Checked the hose from the brake servo to engine for vacuum retention, on the engine side, all good.
5) Rebled the system and found no air in it.

Current Symptoms:
Pedal is solid with the engine off
Pumping the pedal doesn't increase it's firmness
With the engine on, applied pressure will result in the pedal gradually going the floor (over the course of a minute or more)

Any suggestions on where to look from there? Thanks in advance for the help.

 

have you looked at the brake proportioning valve ? ( located under the M/C on the wheel well ) those have a tendency to rust / seize etc....

 

X2. I had the same problem and kept replacing parts till one day I noticed missing paint under the master cylinder at the finder. That valve was the problem.

 

So, it sounds like both suggestions are the pressure reducing valve/brake proportioning valve then?

I had checked that for leaks, there aren't any there, it's dry as a bone. I hadn't considered it further, because I'd thought the symptoms of it going bad involved increased brake pressure to the rear, rather than the pedal depression and lack of braking power I was experiencing. I know RAVE shows only the master cylinder or servo as potential issues associated with the pedal depression.

Is there a way to test the proportioning valve or isolate the system from it in order to confirm? Could I block off the master cylinder itself at the two outlets and then if the same symptom is experienced, know that it's the servo or somehow a bum new master cylinder?

Or is it a cross your fingers, replace the valve and hope you're not wasting money kind of deal?

 

Couple of questions. Have you checked the MC seals on booster end? Is the ABS modulator/accumulator still involved with the rest of the brake plumbing?

 

I've inspected around where the MC plate bolts to the booster and seen no sign of leaks. Is it worth pulling the MC off of the booster to inspect further in your estimation?

The ABS modulator/accumulator is still in the OE setup with not modifications or removals.

 

I would leave nothing on the table, it's brakes. Does the ABS work?

 

I agree. I just wasn't sure that was the next logical step with the MC being new. I'll check it tomorrow though and update.

As for the ABS, I have no reason to believe it doesn't work. The "ANTI LOCK" light follows the typical sequence and doesn't come on any other time. I don't know that the brakes are providing enough power right now for me to road test it and trigger the ABS under emergency braking. Living in LA, there aren't really the conditions to easily trigger slip under heavy braking regardless.

 

The mc shouldn't be leaking but North Korea wasn't tought to have a missile that could hit our mainland for another few years... wet roadway will make em slip. Hopefully it is the proportioning valve and maybe this is a mere coincidence of the previous work. It's hard to troubleshoot through the web.

 

Pulled the MC today and it was dry on the back side, so that looks outwardly to be good.

I wish we could get some rain here to test it on a wet roadway, we could use it. Unfortunately, I haven't felt comfortable enough with the current state of the brakes to take it in search of slippery surfaces for a test.

Is there anything else that would point in a direction other than the proportioning valve? The fact that the descending pedal only presents with the engine turned on? I know web troubleshooting is hard. Just trying to safety net any flawed reasoning on my part prior to making parts purchases.