94 previa 4 wheel abs master and proportioning valve new .low pedal tried line locks on front lines still the pedal only improves marginally

no
rear wheel brakes operate at different pressures because of the proportioning valve in the lines
you are correct that failing brake hoses can fail internally and lock the brakes on but I feel that that is not your problem
This is how disc brakes work
There is no check valve in the master cylinder as there is in a car with drum brakes so any fluid captured by pedal ( the exception here is if the car has read drum brakes in which case that section of the master cylinder will have a check valve )movement is pushed into the calipers and applies the brakes
Now on releasing the pedal , that fluid is allowed to return to the reservoir by a compensating port in the master cylinder
That port blocks up and the disc brakes remain on.
heat build up from the brake pads expands the fluid and so pushes the pads harder onto the disc( Lock on that you release by bleeding the calipers)
SO the problem will be in the master cylinder
having the brakes professionally flushed may relieve the problem for a period but a new master cylinder fitted professionally will fix the problem

Did air get into the master cylinder? If you can, try bleeding the master cylinder. The fact that you aren't getting any fluid out of the line suggests that the master cylinder isn't pumping fluid.

Master cylinders are (usually) on a split circuit system - that is, the front left brake is on the same line as the rear right. The front right brake is on the same circuit as the rear left.

A point to be aware of is when bleeding brakes/doing brake work and if the master cylinder runs dry, is that a seal inside the master cylinder can fail/twist over and not pressurise that particular line, even though it may pressurise the opposite brake line.

I'm in the UK. I have a 1998 Jeep Grand Cherokee 4 litre, which I am slowly rebuilding. Three weeks ago I fitted new front brake rotors, new front callipers and pads. I've been rebuilding cars for over 40 years now and have done countless brake overhauls.

.. and could I get a hard pedal after the job was completed? I must have bled the brakes three times a day for a week and couldn't get a brake pedal. I phoned a full time mechanic I know - he came and tried to bleed the brakes three times and couldn't get a hard pedal.

That's when you think 'The master cylinder was working before I started the job so it must be still working now'.

I fitted a brand new master cylinder and because some parts for older jeeps are hard to come by in the UK, I fitted a second hand proportioning valve. When I bled the brakes again the pedal became rock solid. Fixed.

You're probably fed up with your braking problem now ... and apparently this 'no fluid/brake pedal' is more common than you may think.

Try bleeding the master cylinder if you can. If you suspect that it isn't fully working - try an overhaul kit which is cheaper than a new master cylinder.

As for changing the proportioning valve .. that's your decision. My brakes now work though the truth is I don't actually know which was at fault as I changed both the master cylinder and proportioning valve at the same time. Maybe both were at fault, for whatever reason. I genuinely don't know.

The positive aspect is that you are doing your own work and know what you're doing. You're saving on labour charges.

Pretty normal to not be able to lock the brakes without the assistance of the booster, it'll take a lot of pedal pressure without it. It's also normal for the pedal to feel harder without the booster, this goes for any car/truck with power brakes. Try it with any other car you have and you'll see.

I've worked on several old Cadis but don't recall what year they started using front disc. If it's been changed from drum to disc you'll likely need a different proportioning valve and possibly a different master to get them to work right. Talk to the seller and ask if the brake feel changed when they replaced the master and if they changed any other parts.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/oscar_403cc49e0f00cda7

With or without ABS? Or does the master cylinder have two or four lines coming out it? I have a feeling you have a bad masterclinder. The lines on a non abs are cross proportional, meaning one front and one rear are supplied fluid by one part of the master and the other two are supplied by the other. Try cracking the lines loose on the master cylinder and pressing down on the brake pedal. Try this for ll the lines making sure to tighten the fitting before letting up on the pedal ( just like if you where bleeding the brakes at the wheels). If no luid comes out than you just confirmed a bad mastercylinder.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

if something goes wrong with abs the brake system it reverts back to standard system!! If you can pump brake pedal up and hold it and start engine and it drops thats normal.You say low pedal if you go 15mph on road and check where pedal is when brakes apply.Power brakes will push aLMost to floor while stopped but while moveing you`ll almost go to slide or will go into slide if pedal is pushed to floor.Get on some gravel at 10 mph and hit brakes to a complete stop get out and check to see if all 4 tires slid or 2 front or only 2 rear. this will get you closer to problem good luck.

I've got a 2000 ram had the same problem.... first i changed the master cylinder .... i thought it was the abs valve ... before i spent the $200..... i asked a buddy he said the brake booster was bad changed it and the pedal felt like it did when the truck was new....

Possibly the Proportioning valve. its where the lines go after the master. it has a bleeder screw of some sort on it. The pins inside wear out and stick causing uneven pressure. Replace or rebuild it if that be the case. Also chect the master for fluid leaks at the end of the piston where it connects to the booster push rod. If its leaking rebuild or replace it. 

Does sound like you need a new master cylinder. On a 2000 you wont need to back bleed. You will just need to follow the instruction and bleed the master and all four points. you will probably need to bleed the ABS block and proportioning valve but only if they have bleeders on them.

How did you come to the conclusion that the master cylinder is ok? Does the level of brake fluid in the MCyl. rise & fall while pumping the brakes. I feel no fluid is entering the chamber which has the piston. This piston is directly connected with the brake pedal.Have it checked and the oil seal in it too.....sodeep

Its probably how your bleeding your brakes, but first make sure the bleeding nipples are facing upward. You should bleed diagonaly, front pass wheel then rear driver wheel. Then front driver wheel and rear pass wheel. The abs light has its own computer your have to get it scanned to clear the codes and turn off the light