Why do the brakes keep locking up?

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.

It is quite possible to get a bad (new) part. Sounds as if you have a bad check valve in the master cylinder. I have bought brand new masters which had this problem and torn them down to find that some yahoo left the entire check valve out when assembling the cylinder.Also possible you have a bad brake hose (rubber part).

The master cylinder is mounted on power brake unit (the big black round canister) on firewall held down with 2 12mm nuts depending on braking system usually 2 10mm brake lines need to be removed on out it comes. Bench bleed new master cylinder before install . Fill reservoir with brake fluid and slowly push piston about 1 inch back and fore until air is gone ( assuming mater comes with bench bleed kit ) then install and repeat with someone pressing on pedal lock line closed before pedal is released again EG pump pedal ok hold it (open line watch fluid come out close) ok let up pump it up again repeat may be twice per line. the pedal should be firm once done.

HI
Remove the 2 brake lines from the master cylinder,,,,
Remove the 2 bolts that hold the master to the brake booster...
Remove the master cylinder...
Install the new mastercylinder...
install the 2 bolts removed....
bleed the master with bleed kit that comes with the new master...
install the 2 brake lines..
bleed brakes.....
Let me know if i can help more

I had this same problem on my 90 Jetta but it was the master cylinder that was the problem, I first replaced the master cylinder because it was leaking badly. The new master cylinder I put on was a manufactured defect, the internal piston inside the master cylinder was not machined enough (10mills too long) so when you applied the brakes the piston would not move far enough to release the brakes but after 20 mins they would slowly release themselfs just as you said your brakes do. The only thing that can cause this problem (if you have changed everything) is the master cylinder or the brake pedal itself, did you do anything to the brake pedal or near it to change its movemment ? Make sure the brake pedal once depressed returns fully to the float position where it should be. I would try another master cylinder preferrable a used one from another vdub the same as yours, eg year,model and country of origin some parts are different if it was made in Mexico or Germany. Its going to be a pain in the *** to have to replace the master again but I see no alternative. There is one more thing you can check I just thought of....check all your brake lines from the master to the calipers for any signs of dents or sharpe curves if the metal brake line is bent too much it will restrict the flow of the fluid returning to the master cylinder after brake pedal release but not during braking. Good luck

Sorry to but in BUT

is this a high mileage car from what you discribe new Servo, Master Cylinder, Cylinders and rear brakes.????

Try the rear flexible brake pipe/s OLD RUBBER DELAMINATES well worth the swop. For the cost

360 Volvos used to be good at this one only after a brake fluid change

Old British Mechanic

get ready for some work.you need to remove the master cylinder nuts that hold master cylinder to brake booster.once you remove nuts you have 2 choices remove master cylinder to remove booster. to keep from bending brake lines.or you can leave master cylinder in pull it toward you easily careful dont bend lines .disconnect vacuum line at booster .go inside vechicle to floor on driver side loosen four nuts on floor to booster .disconnect power brake push rod from brake pedal .dont force pushrod to the sides when disconnecting it.remove four booster nuts then remove booster.when in stalling the new booster loosely install four nuts then connect the push rod to the brake pedal .install new clip tighten the booster unit .if you decide to remove master cylinder use inline wrenches on master cylinder to remove lines.also plug the master cylinder brake line holes to keep from losing all brake fluid.

i do agree with the haynes manual.

First off, pb blaster is your best friend for brake line bolts. You want to **** as much of the brake fluid out of the resivour as you can, Then you need to get the brake lines off the master cylinder that is on there. make sure you use the right size flare nut wrench when you are taking the lines off. rounding off brake line bolts is not fun, trust me. once the lines are off, ( turn counter clockwise) Just make sure you have a towel to put under the brake lines. for excess fluid. then with a ratchet, and extension, and socket you need to take the master off the brake booster. Also disconnect any electrical connectors and keep them clear ofthe brake fluid. When you have the 2 bolts out of the brake booster, the master should pull out and then your ready to work on installing the new one. when you get a new ,master, you can either bench bleed it, or if you want to, you can also install the new one, line up the hole in the brake booster, put the 2 bolts back on, then you need to bleed the master. Most come with instructions.

1. Begin by removing the cap.
2. Follow the directions that come with the kit to select the suitable adapters, and then connect the adapters and tubes to the ports on the cylinder. The other ends of the tube extend down into the master cylinder, and are held in place by a plastic clip.
3. With the bleeder kit installed, fill the master cylinder about halfway with new brake fluid. Use a large screwdriver to depress the valve assembly inside the master cylinder. If you do not have a vise you can mount the master cylinder on the vacuum brake booster and bleed the master cylinder.
4. Small bubbles will appear in the fluid. Continue working the valve assembly until no more bubbles appear. This indicates that the cylinder has been thoroughly bled.

once it is bled then you hook up the brake lines, and bleed all the brakes. starting with the furthest from the master cylinder. Passenger rear, driver rear, passenger front, driver front. make sure you check the brakes to make sure there is no air, better to be safe then sorry. also, check anything you had disconnected, to make sure it is not leaking.

In simple terms, by just looking at it, you can figure out how to do the master cylinder, but to be safe, get a repair manual.

Are you having to add fluid? If not replace the master cylinder.

have you replace the caliper? if you did then when you stop loose the bleder of the caliper and if pressure come out tie the bleeder again and haved your mechanic remove the brake master cylinder and see if there is any adjustment for the push rod or shaft behinf the master cilinder and loose the adjustment to make it short but if there is no adjustment then leave the master little loose and if this take care of it you might have a wrong master installed or just need to adjust the rod behind master good luck