Hi, I'm a new member and new owner of a 1961 Mark 2 ( 3.8L, LHD, Automatic). I just posted a brief intro over in the introduction forums - I've owned an MGA for about a decade, but finding the Jaguar has extra complexity I'm not familiar with. I just bought the car last week and have put maybe 50 or so miles on it so still in the shake down phase.

Yesterday I had to do a little harder stop than normal at a red light (not a panic stop, but not the gentle roll in that I've been using). When the light turned green, I could tell the brakes had locked up - there was a squeal and the car didn't want to move. I pulled over, and the brakes seemed already to have unlocked. I was only about 10 minutes from home, so I proceeded on. As I turned onto my street, I thought I could smell burning brakes. After pulling back into the garage, three of the wheels (all but right front) seemed to be putting off a lot of heat, though maybe it was all 4.

Looking at the recent history, JK Restorations in Illinois did work for the seller 2 years ago including master cylinder, front rotors, brake pads, servo (I'm assuming that's the brake servo?), front and rear brake hoses, and new brake fluid. Then a local place a year ago seems to have changed the fluid (DOT 4).

I'm getting ready to go check things out today - any recommendations on what to check? I'm new to disk brakes (on classics). Could this be an expected temporary behavior after a hard stop (seems unlikely, but I'm so new thought I'd ask). I know sometimes the first thing to suspect is the last thing done, but I'd hope that the work done by JK (who I've heard is well respected) would not fall into that category. Seems like the only big thing they didn't touch were the brake cylinders.

 

First, welcome to our wonderful world. Don't worry our cars aren't as complicated as they first seem and we like nothing better than discussing every aspect of them.

How often do we discuss locked brakes? Perhaps it deserves a sticky. There are a few possibilities such as internal collapse of hoses. Since it seems to affect at least three wheels and the master cylinder has been serviced, the servo (booster) is a likely culprit. If it's the servo, either the hydraulic cylinder piston or the air/vacuum piston is sticking. For the latter, I (to the horror of some) suggest a teaspoon of brake fluid poured down the breather (the rubber tube that rises from the servo to a small air cleaner). It usually (it worked for me) unsticks the piston (and it's a very easy thing to do as it doesn't require taking anything apart).

If that doesn't work, you'll need to go deeper with servo and master cylinders. If it does work, then why was the piston dry? In my car it was because the one way valve in the vac line (at the bottom of the vac reservoir) was completely failed allowing a lot of air flow through the servo.

 

I just cracked the code on forum searches to limit to just this forum and and seeing posts similar to mine that seem to point to the brake booster. Like this: https://www.jaguarforums.com/forum/m...mk-2-a-261320/

This is a piece of voodoo equipment to someone that cut my teeth on an MGA. The only booster I have there is to make sure I eat my Wheaties. I’ll try to follow along with the advice there but appreciate any other tips for a newbie or correction if you think I’m on the wrong track.

 

Geoff, I find it's often simpler to Google the whole internet to find info on this forum than use the website search.

 

Thanks, Peter. I see you replied with some of the same info I found (posted by you!) on that other thread while I was searching. I will try that breather trick - seems like a fair place to start.

Point taken on searching the forum, but one issue I've had in searching the web is narrowing the results down to only what's relevant for this car. Mark 2 Mk2 Mkii at least tends to get rid of the more modern variants but then I'm missing relevant info from the 240, 340, Daimler, etc.

 

I have had a similar problem with my Mk2. After a stop, the brakes did not want to release until some seconds had elapsed.
After much head scratching I concluded that the air valve piston in the servo was sticking.
This particular servo came from a well known supplier of Jag parts and had been in use for only a few years.
I took it apart and found it was made to a very poor standard and not at all like an OEM design. At a guess I would say it was a third world product.
I had been tempted to get a rebuild kit to fix the problem but I was very doubtful that would have been satisfactory given the poor style of manufacture.

I bought a new servo from SNG Barratt and was very pleased to see that it was made just like an OEM one.
It solved my problem and has worked well for two years.

So I think that safety rules here and you should get the best quality product.

I have attached a document on how the servo works, that I found useful.

Bruce

 

Bruce, thank you for that document! That will help me demystify and probably make for better diagnosis if this continues.
Peter, I don't seem to have a breather hose or filter. Are we talking about the bit circled in the attached picture?

 

Here is what I see from above. To me, it looks like the whole thing is mounted sideways - the air valve cover is pointing back to the engine instead of up, and there is no air cleaner hose attached, or even it seems a place for one.

 

I've had caliper pistons seize on an XJ6, (rears), but the piston design is different, being more modern, but worth checking IMHO. The early caliper cylinders had a funny little clutch thing that was designed to withdraw the piston after pressure was released in the hydraulic line. In your post you mention a lot of brake components have been attended to, but not the caliper pistons.

 

Lock up is usually Booster related or on Automatic cars the rear hill holder device not working for a myriad of reasons including the throttle switch.

Also be aware of Caliper Over Return on you brake journey & low or soggy pedals.

https://www.jaguarforums.com/forum/m...ds-etc-211186/

 

This is the correct orientation for the brake servo as seen on my 1968 S Type before the engine install.

 

That automatic anti creep system only works on the rear brakes. I have encountered the problem of them sticking on a couple of times on automatic cars. As Glyn says the throttle switch is usually the culprit.
In this case the front brakes were heating up, so it is basically trapped hydraulic fluid in the system.
The suggestions of brake booster or sticking master cylinder are most probable.

 

Geoff, I think your servo has been replaced. Apart from looking new, it's a slightly different pattern from the OE one in my car. As more recent designs replaced the air/vac piston with a diaphragm (and had no need for the air cleaner), my suggestion isn't applicable. It's now a case of going on to the other parts of the servo and most likely buying a replacement of known quality.

 

Thanks, Peter. The servo was replaced about 2 years ago by JK Restorations. I’ll try to call them to see which unit they fitted to help determine how to proceed.

Are there clear recommendations for a high quality replacement unit if it gets there?

Bruce recommended SNG Barrett as a source for servos, and they seem to sell two. One looks similar to this one and the other is only available if you send a core.

Moss Motors (the big gorilla for MG parts in US but maybe not so for Jaguar) seems to carry one that is rebuilt original (or designed the same) but a core is optional.

Geoff

 

Barratts have a good reputation, have a base in the US, and deliver promptly. They may not be the cheapest, but might give you discount if you tell them you're a member of a Jaguar club. Also, if the product isn't good, they try to put things right, refund or replace. For brake or other safety critical parts, I'd go to them. For the choice of item, give them a call and see what they suggest.

Are you going to do other checks first to be absolutely certain the servo is the problem?

 

Yes, I do plan to investigate to determine the actual problem. I don't mind spending to fix a problem right, but I don't like spending to guess at a problem.

The challenge at moment is the brakes seem to have released themselves. My first step yesterday was to go out on a controlled drive down our street and everything seems normal again.
If there is some bench check I can do, let me know. Apart from that, I am trying to prep for quick diagnostic checks if they do lock again. I'm trying to get a game plan for quick diagnostic checks. Here's the rough thinking for now:

1) Confirm that the front brakes are also locked. Hoping I can visually see the caliper/pad position from outside. This would eliminate the automatic hill check mechanism (right?)
2) Check that the master cylinder is returning back to the relaxed/unloaded position. I need to eyeball the MC better here, but I guess I am just tracking that the pushrod is fully extended?
3) At this point, it would seem to point to "servo". I'm not clear what steps I could take to further diagnose within the servo, but open to suggestions.
4) I do plan to look for collapsed brake hoses, but hoping since they were all new 2 years ago that is not the problem.

 

It's hard to see if the caliper is clear of the disc, I'd put the car on axle stands and see if the wheels turned by hand. There's no need to move the car. It's enough to run the engine, give a hard push on the brake pedal and hold it for a few seconds. If the brakes slowly release after switching the ignition off (as air leaks into the vacuum system), the fault is most likely in the servo.

 

I am sure that SNG do two brake servos. The first is basically a swap of your own for a rebuilt unit and is like for like.
The second unit is a brand new replacement and comes with a higher compression ratio of 4:25:1. Someone will come up with the compression ration of the old unit but in short this one means if you put your foot on the pedal you don't have to push so hard to get the same result of locking your brakes.
To be truthful I fitted this new uprated unit some five years ago and I could not feel any difference in normal driving conditions. I don't tend to stamp on the brakes but it is nice to know that if it was needed then it is there.

 

Jaguargeoff
I do not think that your brakes are locking on because of the anti roll back solenoid valve since this only works on the rear brakes.
Just to reiterate, the rebuilt servo that is offered by SNGB is a repair of an original servo that was fitted when new,
This is characterized by the octagonal style air valve cover.
From your picture, you do not have this style but a later one made by Lockheed that is sold as a replacement. P/N LR18230.
Therefore in my book you can only buy the LR1820 since you do not have a core to return.
Also I was not talking about the air filter that you circled but about the air valve piston



Bruce

 

modern disk brake calipers are designed, AFAIK, to release the pads from the rotors by the minor amount of pressure exerted on them by friction from between the pads and rotors when external pressure is relieved when the brake pedal is released. if not used for an extended amount of time they can stick. i would drive around a parking lot a low speed for a couple of times while constantly applying the brakes and see if that doesn't free them up. even then, i would be looking for any signs of excess heat for a few hundred miles or so.
if that doesn't work i'd probably be taking them apart with the expectation of rebuilding a cylinder or two.