For many years I drove on drum brakes particularly in MK7s and I did not have any major problems in pulling up. I am of the opinion that a well maintained set of drum brakes is nearly as good as disc brakes however they do have a problem with fade when hot and are darn near useless after being immersed in water while crossing a flooded causeway.
I was happily driving the metallic grey MK 1 which is equipped with drum brakes and knocking up a few Ks when I noticed an appreciable decay in braking effect and the pedal was getting closer to the floor. I almost ignored it as I am getting close to completion of the MK2 disc brake front end which is going to be fitted in the near future. However it is not in my nature to leave a vehicle in a "suspect" state and I also heard a couple of clacks from the front end while pulling up in the shed.
Getting my dearly beloved Honor to actuate the brake pedal while I roamed around outside revealed that there was obviously a fair bit of movement going on in the brake system in the front end. Jacking up the car, removing the front wheels and brake drums revealed brake shoes that were in very good order and brake drums that were smooth and shiny without any scoring. The previous owner had advised me that he had the brakes "done up" only a short time before I acquired the vehicle. When I was preparing it for a "roadworthy" (safety certificate) I had checked the brakes and the shoes and drums were in apparently first class order and they were also checked and passed by the person carrying out the safety certificate inspection.
On close inspection I noted that the automatic adjuster bar was at the limit of its travel. Hold on – what is going on here thought I!. I checked the lining thickness which was the correct ¼ inch and also noted that the trailing shoe on the right hand front brake did not show any sign of contact with the drum. In fact with the brake drums removed you could put your foot on the pedal and get brake pressure even though there was theoretically nothing to push against or to stop the slave cylinder pistons flying out of the slave cylinder.. It turned out that the brake slave cylinders were at the extremity of movement and locked up against the travel limit of the automatic adjuster bar.
The correct internal diameter of a MK 1 Brake drum is 11 1/8 inches. I checked the internal diameter of the drums on this car and found out that one was 11 5/16 inches and the other was very close to 11 3/8 inches. I.e. one was oversize by 3/16 inch and the other was nearly ¼ inch.
The person responsible for turning out these drums to such an amount and refitting them is guilty of criminal negligence. While I have been unable to find out any recommended limits by Jaguar, general automotive industry consensus is that on a drum of this size around 1/8 inch or about 125 thousandths of an inch (.125 inch) oversize would be the limit and after that the drum should be discarded. The problem is that a very hot thin drum under severe braking has the potential to separate the drum brake contact area from the brake drum face assembly leading to a total loss of brakes. What made matters worse is that they had not bothered to consider putting in thicker linings which are readily available by specific order.
I have all sort of good Jaguar things hidden away in the shed at the end of the stables which I have christened "Possum Palace" as a family of possums lives there as well. They have claimed their own paw paw tree which is regularly raided and I can't use the top shelf of the Dexion shelving as they use it for their daytime sleeping site. Anything stowed on the top shelf is promptly kicked off to the detriment of the car parked in the shed! Anyway back on to the brake problem. I found that I had four MK 1 drums which varied from .040 to ,080 inches. oversize and I selected the best two which got a quick hit from the angle grinder cup brush to clean up the light surface rust in the drum contact surface areas followed by a clean up with some 280 grit wet and dry paper.
Resetting the brake shoes and wheel cylinder pistons to the inboard limits of the automatic adjusters and fitting these "new" drums resulted in only a small movement outward in the automatic adjuster bar and the brake pedal returned to a maximum height. The differences in a road test were quite dramatic as I was able to lock up the front wheels on bitumen in a panic stop from 60ks/hr.
My advice to anyone overhauling early model drum brakes [which ceased in Jaguar production in 1959] is to speak to a brake specialist and ensure the drums are properly measured and reasonably legal and that appropriate thickness lining are used to re-line the brake shoes to account for any oversize drums. By the way the correct composition linings for a MK 1 drum brake system are quite soft and the experts at Action Brakes in Nudgee Road, Hendra suggest a maximum life of only 20,000 miles in normal use.