1979 Mark V Brakes Overheating

Hi Bill,
Thanks for a great website.
I have a 1979 Mark V. A beautiful car, white without a vinyl roof (factory delete option).
My car recently came from a private collection in the US to Australia. A nice car with 73 000 miles on the clock. All these miles were done in the first 20 years or so of its life and then for the next 10 years it sat in a collectors building in Washington State. I had all the usual things go wrong due it to sitting still for years and solved most of them by, a distributor rebuild, carb rebuild, new front wheel bearings and of course, a full brake rebuild. This included a new master cylinder, reconditioned proportioning valve, new hoses and overhauled calipers and hardware. Disc run out was within tolerance. Brake linings were also new.
I have a full set of manuals and a friend of mine, (the best mechanic in town) and I set about the brake rebuild. We followed the manuals word for word and the end result is pretty good. The brakes work well. However, the rear brakes run a bit hot in use. Not so hot they smoke or impede the movement of the vehicle, but hot enough to be hotter than the front wheels by 10 to 20 degrees centigrade. (I have a heat sensing tool). The rear axle bearings do not run hot, we also changed the rear axle oil. Additionally, I checked the park brake cables and it releases fine.
We dismantled the brakes again only to find nothing obviously wrong and reassembled them. We still have the same problem, the rear brakes get hot. The rear wheel hub gets up to 63 to 73 degrees centigrade, wheres the front wheel hubs only get to around 50 to 60 degrees centigrade. 65 degrees centigrade sure feels hot to touch. When the car is on the hoist there is more drag on the back wheels caused by the brakes than the front wheels. When driving, you can hear a feint squeak coming from the back brakes.
Back to the manual I went and retraced our steps. By process of elimination the manual guides me to the rear caliper piston/adjuster assemblies. The manual states that if, “the rear brakes over or under adjust, replace the whole piston/adjuster assembly”.
Have you encountered this problem before?
It seems odd that both rear calipers get warm by the same amount. It is almost like there is a small amount of residual pressure in the rear brake lines holding them on. I used a reputable brand master cylinder that does not have a check valve if used for a rear drum brake system We even moved the master cylinder forward 3/8ths of an inch off the pedal push rod to ensure it was releasing any residual pressure. No pressure was there to release as there was no change to the rear caliper drag.
We intend to dismantle the rear calipers again soon and inspect the piston/adjuster assemblies even closer.
Any thoughts ?
Thanks for a great website. I find the technical parts diagrams very useful along with your parts supply.
Regards,
Alasdair Webb
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Hi Alasdair –
I can understand your concern with overheating brakes if they are indeed overheating. The question is are they actually overheating or not? We have no temperature specification charts to refer to for comparison. If we suspect a vehicle to have overheating brakes we drive the car to operating temperature and test for “brake drag”. This can be done on a safe level smooth surface with the car moving very slowly. At this time you shift the transmission into neutral and observe the car as it rolls to a smooth halt without any stopping force from the brakes. Then the car is manually pushed in neutral and must resume movement easily and then roll to a slow easy stop once more when you stop pushing. If the above is correct the brake system should not be causing any overheating. I assume that all of the parts that you have installed as well as the assembly of these parts is correct. We have encountered similar problems in the past with incorrect or rebuilt master brake cylinders used on these cars. Our records do not indicate that a brake master cylinder was purchased from us therefore I would advise you to recheck that a correct new unit was installed. I hope that the above information helps. Please don’t hesitate to contact us for further advice in this matter.
Sincerely,
Bill