XJ XJ6 / XJR6 ( X300 ) 1995-1997

Hand Parking brake poor weak performance

 
  #1  
Old 03-07-2019, 02:07 PM
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Default Hand Parking brake poor weak performance

My 94 manual XJR failed MOT test on hand brake efficiency. This is tested with the car going forwards. I say this as it works fine in reverse and we even spun it round and put it on the rolling road backwards which showed a vast improvement and a braking efficiency that would have passed with flying colours in the forward direction. The brake is single leading shoe so should work equally as well in both directions. I have just fitted new shoes. The shoes are all identical and just need to be fitted the correct way up as the bottom pull off spring is longer than the top one.
It is possible someone has adjusted the cable rather than the screw adjusters inside the drum so first thing I will slacken off the cable and re-adjust on the individual shoe adjusters and then re-adjust the cable. I may fit new discs as well as the internal drums are scored due to there being no linings left at all. It may be possible to turn the actuating mechanism through 180 degrees which may transfer the better rear directional brake force to the forward direction. This should not however be the solution!

Any other thoughts on this one?

Thanks
Charlie
 
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Old 03-07-2019, 03:11 PM
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I‘d go with the new discs and reset every thing to factory before trying to ‘modify’ what Jaguar did, it worked when new so returning back to standard should do the trick.
 
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Old 03-08-2019, 02:23 AM
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Originally Posted by Atco View Post
...... I may fit new discs as well as the internal drums are scored due to there being no linings left at all. .....
First thought is the tester is being especially pedantic.

It must have been caused in the past by the parking brake being ON for some time when the vehicle was driven. As it is intended only to be used (except in emergency) when the vehicle is stationary, there should be little or no friction wear on the shoes.

Two main things that effect parking brake performance are:
1. lack of use (too many drivers rely on the parking pawl in the transmission to hold the vehicle) and the mechanism seizes
2. cable stretch

If the wheels can be locked with the adjusters then you should not need to replace the discs. Cable stretch is often more than can be taken up with the yoke adjuster and you may need to look at replacement.

Graham
 
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Old 03-08-2019, 03:59 AM
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Originally Posted by Atco View Post
My 94 manual XJR failed MOT test on hand brake efficiency. This is tested with the car going forwards. I say this as it works fine in reverse and we even spun it round and put it on the rolling road backwards which showed a vast improvement and a braking efficiency that would have passed with flying colours in the forward direction. The brake is single leading shoe so should work equally as well in both directions.(...)
This is not entirely true. Generally, when it goes wrong, parking brake in x300 is a small devil's child, there are plenty of things affecting it and it's a right [email protected] to set up right, took me months on my own car.
First is obviously the cables, if these are not completely free(as in you should be able to pull cable with your hand on each side and you should feel the pads moving easily back and forth) then you'll need to replace them. Annoyingly they are much more expensive if you compare them to other Jag models.
Second. parking brake calipers should be as free to move as it's possible, simply remove and clean until they move freely.
Third, parking brake locking pins normally are attached to backing plate, if the holes in them are rusted and allow excessive movement, forget about setting it all up. Use bolts instead so they sit fairly tight on plate.
Fourth, you have to set up star adjusters with loose cables. Adjust each until wheel is fully locked and then back off slowly just to free up wheel.
Fifth, connect center cable, tighten it up until wheels are locked in D and back off until they just start to spin again.
And here's the annoying bit. At this point it's more than likely that one wheel will be locked and the other moving fine. With the rear end on axle stands you should be able to find a sweet spot where both wheels spin in D and both lock in D or R when you apply parking brake but mind you, adjusting one side star adjuster by even a 1cm up or down will annoyingly affect the other side so you'll have to move from one to the other side until both sides work together and not against each other side and only then adjust main adjuster again
Also, if your a-frame bushes are shot and allow excessive movement expect parking brake locking on road bumps and you'll need to further adjust linkage under the car.
 
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  #5  
Old 03-14-2019, 06:42 PM
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I have used the same MOT station for 18 years and have a good working relationship with them. I could not argue they were correct to fail it.
I have now rebuilt the handbrake again and all seems good. Thanks for all the comments. (New discs fitted as well although measuring the old ones they were 180mm internal diameter. Max diameter is I believe 181mm)
The actuators/expanders can not be turned round.
I stripped all the shoes off again and dismantled the expander units from the end of the cable. I disconnected the cable where it goes around the horse shoe which allows left to right compensation. Once the cables are free you can oil them from both ends working the cable back & forth to encourage the oil to pass right along the cable. Also expanders lubricated with a smear of Keenol grease/expander lubricant.
Normally on a mechanically operated brake you want the mechanical links to be at 90 degrees to each other when the brake is on which gives you maximum force.
The expander mechanism in this brake needs setting up differently. As the cable pulls the expander apart, the further you pull the cable more movement of the expander takes place but the angular rotation of the expander link is not in direct proportion to the angle of the handbrake lever in the car.
The maximum force you can apply to the shoes happens on the initial pull from the rest position because the relative movement of the expander at this point is small in comparison with the angular rotation of the handbrake lever.

So my recommendation is:
Fit the shoes with springs and locking pins. The shoes at the bottom are on a “stop” at this point. Reconnect the cable in the centre of the car greasing it to encourage it to slide around the compensator, and adjust to the point where the shoes are just about to lift off the stop. i.e. take out the slack. Lock off the lock nut if so fitted and then refit the discs. Then adjust the star adjustors at the top between the shoes to just give rotational clearance. Note point from previous post about suspension movement.
If you adjust the handbrake using the central adjustor on the cable you will be expanding the expanders to a position where they become less efficient. Adjustment should always be done using the adjustors between the shoes.

On the rolling road the hand/parking brake performance recorded was 307kgf & 341kgf for left and right wheels. Foot/service brake recorded 266 & 264kgf.
16% of 2000kg is 320kg so I am assuming a minimum of 160kgf on each rear wheel on the hand/parking brake would be a pass?

16% is the required hand/parking brake efficiency required for UK MOT.

I have not seen an original Jaguar set up process but this worked for my car.


XJR6 brake test results
I hope this will be of help.
 
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Old 03-14-2019, 07:08 PM
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That would be a first for Jaguar, the handbrake being more powerful than the foot brake
 
 
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