The XJ doesn't see much action in the past months. Took it for a drive, during which front brakes started dragging more and more to the point I was barely able to reverse it up the drive way in the end.
Didn't crack the bleeders at the time to see if that would free them up since I didn't know at the time this could be used to narrow down the source of the seizing up. After cooling down, the wheels were able to spin albeit with quite some resistance.
Some time ago this happened with the left front, heated up to the point the centre cap literally melted off.
Replaced the piston and rubber seal on that side, both rubber brake lines had already been replaced before that when I tried to solve a spongy brake pedal where replacing the master cylinder had not solved that issue (took the MC out again and returned).

About to do the other piston as well (for good measure) but since now BOTH wheels seized, should I look into a more 'common denominator'?
MC? Proportioning valve (although apparently that is rare to fail)?.
The brake fluid shows some very fine black material on the edges in the MC, sign of seals degrading?
THe brake booster, although it makes a "whooshy" sound when brakes applied (it has done this ever since I got the XJ 10+ years ago), seems to pass all the tests I can find (brake pedal firming up with engine off, going down only slightly when engine is started, etc)

Keen to get some input, since reading up on it, it doesn't necessarily match symptoms of a bad MC or proportioning valve (although regarding the PV, I had one occasion long ago where my rear wheels locked up breaking whilst going down a slight hill in wet conditions). Discs in front, drums in the rear.

Thanks in advance

 

Well, here's the thing... They're both doing it... So cracking the brake line open would confirm you've got residual pressure which you already know... So break it down... Both rubber caliper lines went bad at the same time ?? Highly unlikely..
Proportional valve hanging up ? Possibly, but again unlikely if it was bled properly ... Here's what I'd be checking : Make sure the "new" master cylinder has complete travel when the pedal is released... Measure what the inside clearance of the master cylinder is at rest and check the booster rod to make sure it's not too long and not letting the Master cylinder return completely to reset position... If it is in fact too long guess which set of wheels lock up... Yep, the front...

 

MC wasn't replaced. I put the OEM back in, but that was years ago. With no issues untill now. Unless the booster rod has moved out by itself over the years but.. is that likely?
Brake hoses have less than 40 miles on them.

Only bleeding that took place, was after doing the left front piston. I drove it around after that and it seemed OK. Two person bleed but not with PV bleeding tool inserted (never heard of it or hear anyone mention it untill I started reading up on proportioning valves)...but no brake warning light on after that either so I assume PV 'thing' hasn't shifted

 

Forgot that I already had started a thread for this.
Doing too many nightshifts here.

 

What kind of piston does the caliper use,steel or phelonic? I had trouble in the past on ford ranger front wheels locking up.Ended up changing the whole caliper out to resolve the issue.Hope this helps.

 

Can't find the original piston in my garage, but the replacement is a Centric from Rockauto -to answer your question

 

Are both the front brakes hot? Check with an infa red thermometer? My knee jerk reaction is bad calipers, I've gone nuts trying to rebuild some bad ones, and my experience is much easier to just replace them. Once that boot gets rust on the backside it's gone. But if both front really are dragging and not just one, Id go back to Corky's post

 

Both were hot, I'd say equally. Ot tge left/right difference as before. Didn't take the gun out on them that time.

Even the rebuilt one seized up again now so I thought it wasn't at the level of the
calipers. But wouldn't be surprised if after the other causes, the calipers will end up being replaced eventually..

 

Divits worn into the pad guides can cause the pads to not back off and thus drag when brake is released. this can cause a lot of heat, which in turn can cause further problems. Inspect the pad guides. if divits are worn into the rails, then you need to repair, or replace them ( knuckle is replaced). Repair involves welding to fill the divits, then grinding back down smooth to restore original surface. Mine dragged do to divits, but caught it before it got real bad. repair was quick and easy, and can be done with knuckles on the jeep or off. hope it is easy fix for you, what ever it be. but give the pad rails a look, it dont take much wear to cause a problem.

good luck

 

7

I have found that a coat of brake grease on the piston and bore outside of the brake fluid wetted surfaces, inside the boots, will go a long ways towards preventing corrosion. Even new replacement cylinders and calipers may lack this grease. I always check and add as needed when installing new ones, and always include the grease in my own rebuilds. since I learnt this trick, I have not had corrosion problems under the boots on any of my cars. Only use brake system compatible grease made for this purpose. Other non brake system compatible grease WILL contaminate and destroy the brake system rubber bits.

 

I would check the mast cylinder to if it is sticking and not returning or maybe the booster is not returning .