Nissan Mistral brake issue

I have a 94 Nissan Mistral--a RHD-- that has a brake issue I'm trying to find. The pedal intermittently loses pressure and had to be pumped so I can stop. 

Obviously, I have found this issue. It's been to two shops and they haven't found anything. All drums, pads, shoes, when cylinders, hoses, calipers, rotors have been replaced twice in the last year. The master cylinder had been replaced twice as well. The master cylinder reservoir is full of brake fluid. There's no obvious leaks on and brake lines. 

Yesterday there was a thump and now the it squeals when it drives. May be unrelated though.

Anyone have any ideas?

sounds like a failing master cylinder to me.  quality of aftermarket parts sucks these days.  i have had brand new master cylinders that were bad straight out of the box.

when does it happen?  ie after a full-throttle blast, only when going uphill, low speed vs high speed, when vehicle is cold vs fully warmed up, etc?

In reply to AngryCorvair (Forum Supporter) :

I've actually replaced the master cylinder twice this year as well. It does it more when it's been driving awhile, but there isn't really any other consistent thing that I can pin down. Honestly, I may replace the master cylinder again to try it, but it's been a crazy ride

just some ideas here, no specific experience with your vehicle.
 

Check where the brake lines run through the engine compartment and near any exhaust components. A missing heat shield,  bracket or replacement line routed next to exhaust could let one do the lines heat up enough to cause weird behavior. 

How are the wheel bearings? Any change they have enough play that the rotors are knocking the pads back on long stretches without brakes being applied?

Any chance the master cylinder of brake system has a residual pressure valve that's failed, or is missing after all the replacement parts were installed?

Are there multiple equipment levels on that truck or just one size fits all?  Also, check for a factory brake bleeding procedure.

I'm in the process of bringing an 83' Civic back to life, but the car is old enough and rare enough that brake system parts aren't well labeled.  The base car used a different master cylinder and calipers, which someone had attempted to install on my car.  Couldn't get brake pressure for the life of me.  Then I looked a bit closer and found it had the wrong calipers, which had too much gap between the pads and rotors, along with two right side calipers instead of one left and one right.  Once I found the correct parts with the right clearances things were much better, but it was tough because my originals were gone.  I also found an old factory service manual which detailed a strange order for bleeding the brakes.

So all of the parts I put on match the OEM numbers. I have a website that has all the schematics and part numbers for my truck. They all match dimensions and look of the parts I removed. 

There's no visible leaks, but I'm sure one's still possible. 

I've tried finding a service manual but have had no luck. Since it's a JDM, I've only found Japanese manuals. If anytime has somewhere I could look for one, that would be great.

I'll check in the other ideas this week

The pad knockback theory is a good one. If this is happening, it would happen far less when cruising steadily in a straight line, and more when cornering and braking. Pump up the brake and then give it a left & right swerve and then a good hit on the brakes. If it needs pumping up again, and it doesn't need pumping up after lots of straight-line cruising, that suggests pad knockback, which is caused by worn wheel bearings (or just running huge brake discs, but a Mistral would need a lot of mods to have room for those).

The temperature issue is also worth investigating, if this is the issue it won't happen at first when you start the car from full cold and will happen faster and faster as the car approaches normal operating temp.

Once you eliminate those two, crappy new MCs will be the top suspect again. The only other possibility might be an ABS accumulator failure.

Edit: Or maybe an oddball brake booster failure could be another possibility?

Does it have abs?  Maybe something going on there if so...

Are the calipers on the correct side?  You will never get the air out if the bleeder screw is at the bottom of the caliper.  Are there bleeders on the master cylinder?  Is there some strange high spot that will trap air in the lines?  Do the wheels have to be loaded (as opposed to hanging off the ground) to bleed the system?  Look for a brake proportioning valve that is attached somehow to the suspension.

If none of that works, find someone smarter than me.

I've had something similar on a Vanagon. Found out the proportioning block for the rear brakes was corroded  enough that it would bleed ok but always had a soft peddle you had to pump to brake. Possible crimp on one of the hard lines might be a issue as well. 

In reply to GameboyRMH :

This is the theory I'm trying next. I've noticed it does seem to happen more when cornering. I hadn't thought of it much before because three different shops said it wasn't. But they also said the fuel filter wasn't bad. And the sensor for it wasn't even plugged in. 

Long story short, I replaced the filter myself and I'm having the bearings done tomorrow.

My Expedition had a soft pedal occasionally, but only with the winter tires on.  The winter wheels were the wrong offset and caused the pads to get pushed in some.  This also means the wheel bearings must have been deflecting.   Kids totalled that one and fixed it for me.