The master cylinder brake lines broke before the proportion valve

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

What model master cylinder have you installed?

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.

to reset the proportioning valve try bleeding the master by applying pressure and slowly opening one line at the master until the valve centres. you may have to do the same with the other line to get it to centre, sometimes you have to hit the area where the valve is as it may be stuck to get it to move.

master cylinder have 3 sections. front, back and proportion valve.The valve control the pressure to the rear brakes.Start bleding from the master cylinder. if the pressure is good inspect the line for damage.if one line don;t have pressure replace master cylinder
Good luck

It could be your break line is clogged with air bubbles or your brake pads/shoes are contaminated. If not that I'd say you need to replace your master cylinder. You can buy a used master cylinder online for pretty cheap.

check for fluid before and after the proportioning valve by disconnection the line after the valve and applying the brakes, if no fluid after valve try before valve if fluid is there then it is a bad proportioning valve. you did bleed the master cylinder before connecting the brake line and trying to bleed the brakes all at once right? If the master cylinder was not bled first then a lot of air may have been forced into the rest of the brake lines.

the stratus is know for clasped rear brake hoses .This stops the flow off brake fluid. try opening the brake lines at the master cylinder that go to the rear.pump up brakes first! if you got fluid than check the brake line if this is rear disc brakes on the line at the rear will be a built in proportioning valve.if drum loosen rubber hose line before the rubber part to see if you have fluid. If you do replace the brake hose! On you 74 challenger by the way i love dodge! any way on the valve there is a small button on the front of the proportioning valve that pops out it needs to be tap in lightly. also hit the brake peddel kind of hard a couple off times if that doesn't work. thanks for your question


p.s that hex bar on the is a junction not a proportioning valve .

Sounds like all your basic brakes functions are covered. I ran into this intermittant problem before, it turned out to be the brake proportioning valve..it's that block type valve with the electrical connector in the center. The steel brake lines from the master connect to it. The valve regulates the pressure between the front and back brakes. The valve maybe malfuctioning internally.

check brake lines they may have gotten pinched very commonok new c/k trucks when you crank the torsion bars up to far you may have hit something and pinched the brake line check those before messing with the master cylinder those are a real pain