Been reading up on it, and now after some opinions on this.

Background: the xj has been sitting a lot the past year. Previous drive around the block, left front wheel barely able to spin towards the end, wheel cap literally melted of the wheel, hanging on by some melted strings of plastic.

Rebuilt the calliper piston on that side. Already had replaced the rubber brakelines both sides some time before that.

Back to now:
Didn't get to drive it till recently (waiting for new alt). Test driving around the neighbourhood for 15 min, towards the end both front wheels severely dragging, only just made it back up the driveway. Just.

Lifted the front thinking it must have been the right front wheel sticking since I had not done that piston yet. However both wheels had minimal movement.

Let it cool for 2 hrs after which both wheels showed some movement again. It was dark outside and didn't crack the bleeders at the time to see if that would free them up.

So am I looking for a 'common denominator' for both wheels seizing up now?
Would a bad master cylinder cause this?

Doing the piston on the left side didn't prevent it from seizing up again this time and rubber brake hoses have already been replaced.
Time for new calipers all together? Probably check tomorrow if the pistons go back in "manually".

Ps: some more brake backgrounds

I have ALWAYS had a spongy break pedal.
Replaced master cylinder a few years ago which made no difference to the spongy brakes , so swapped the OEM master cylinder back in and returned it. Hissing sound when applying brake has been there for years, but these are issues not related to the seizing brakes I thing, although I'm happy to hear any ideas about that as well

Cheers

 

Its probably the master cyl, but may be a steel lines or proportioning valve

It also may be grooves worn into the steering knuckles, I presume you have inspected for this, all my XJ had grooved knucles which needed welding up

Its the same price to order a new MC complete from Rock Auto as buy a MC overhail kit from Australia

In addition, older MC often have worn or pitted bore

I have a spare MC obtained off a wreck, local sellers have them sometimes on Gumtree

 

When I did the left front piston I only had a small groove on the caliper which I just smoothed out. Not welded anything. Yeah, I've read and heard it mentioned; seems a bit drastic to me really. I reckon by the time those grooves are so deep they actually need welding, you might just as well replace the whole thing. My groove was quite small, wouldn't know what to weld there.

 

In most cases I have worked on, the M/C piston was not being allowed to return all of the way. This blocks the SMALL passage back to the reservoir. The fluid remains under some pressure in the lines and causes your result.
That "hissing" sound concerns me. Something is up with your power brake booster. It may be what is not allowing your M/C piston to properly return to the "at rest" position.
Verify that you have some brake pedal "free play."

 

I've had that hissing sound for as long as I can remember.. so long before this occurred, but that's why I years ago tried another MC to see if that would fix it. Only when pedal is being pushed

I agree it does not sound normal, Ive checked that pedal and did that over yhe past years and seems normal; I have no symptoms of a bad booster either. Thought maybe booster check valve? Plan to add that to the order once I worked out what needs replacing.

 

Excerpt from FSM

If brake drag occurs at all wheels, the problem may be related to a blocked master cylinder compensator port or faulty power booster (binds-does not release).The condition will worsen as brake temperature increases.

Based on that, I would try the easiest two things

1) Dismantle the compensator valve
2) Try and ease the master cylinder free of the booster, which may be possible without undoing all hydraulic lines, but if you are doing the prop valve, a bleed will be needed anyway,
check for obvious corrosion around the plunger and MC/booster junction, which is not uncommon, as fluid leaks often take place here on older systems

If I saw corrossion, I may be tempted to spray Inox or similar into the area to hopefully free up any sticking parts

Ultimately, in my experience, a braking system does require a "parts cannon" approach, I have an over 50 year old brake system on one of my other cars, and its had a virtually complete overhaul by me, including rebuilt booster, re-sleeved master & calipers, new pistons, seals and rotors, new hoses and some steel lines, park brake overhaul, brake switches

If you get stuck on the road, I found loosening the appropriate master cyl union allowed me to get a few more miles down the road, then do it again etc etc, got me home, was a bad hose

 

Yeah the parts cannon is why I'm trying to diagnose it as best as I can. I know the compensator channel but not the valve. What do you mean with dismantling that valve?

 

Before getting too deep into the master cylinder might want to check the master cylinder push rod adjustment. It's easy and it's free

 

The prortioning valve is just downstream of the master a cylinder, a rectangular shaped valve, its in the FSM

That sounds like a 1st class idea, did not notice a description of how to do that in the FSM, but I only had a cursory look ?

 

Here ya go.

 

ah the proportioning valve. In your post #6 you stated to 'dismantle compensation valve'

 

Looks I need to remove the MC anyway for adjusting the rod. I'll give it a read after some sleep here

 

did you see the PDF in the post?

 

Yes I did, thanks for that!
Thats what I meant with giving it a read. I think the 97 rod looks different but the procedure might be the same.

Edit: I might be overlooking it but can't find how to adjust the rod if needed. Lots of info and measurements on how to determine the correct depth.

 

Also keen to hear what your method is