Brake problem , how hot should the hubs get?

Saloons
1

Ok we went for a spin in the Jag , found a nice pub about 10 miles from home , about 5 miles on the way back , a faint screech started , now and again , like the sound of a fan belt slipping , or a water pump on its way out !
So kept my eyes on the temp till we got home , all was well , no water from the pump , fan belt was ok .

So thought about the brakes , so feeling all 4 wheel , they was all very hot , too hot I think !

I did notice that when I did use the brakes , the servo would hang on for a few seconds after I let go of the brake pedal , but thought nothing of it , maybe the servo is hanging on a little I thought .

So yesterday I disconnected the servo , and took the car out for a spin , well I know the servo is working , because of the amount of force needed to stop the car ,

After around 15 miles I got back home , and felt the hubs , the 2 back ones where hot as the day before , but the 2 fronts where a lot cooler , normal I would say !

Had a talk to my dad and he said it should be the other way around , as the front brakes do more work , he asked me about a brake balancer , on his SP250 there is a device that splits the pressure , more to the front then back , well as far as I know the MK2 , has not got one .

So , what do you think ?

Have I got 2 problems , the servo , as when it was disconnected the hubs at the front are normal temperature .

The brakes must be holding on , but the car rolls fine , slight hills I have to use the brakes , and car rolls fine , I can push it with no problems , as I roll it up to the garage door , so it can’t be opened ,

In the manual it says master cylinder by-pass port chocked , for brakes holding on

Am thinking I should get a new servo , one of them 4-25-1 jobs , and maybe a master cylinder while I am at it both are standard from 1968 , both have been rebuilt in the past thou !

Brake system is , air tank removed from the servo line , copper brake pipes , 3 new rubber brake hoses , pistons in callipers rebuilt around 3 years ago , all new brake pads , silicone brake fluid.

Anyone else had the same kind of problem ?

2

The servo air control valves sometimes stick on after applying the brakes. They will often release after a bit (a few seconds). This would explain the difference you observe between running with the servo connected and disconnected, and why the wheels spin/roll/car can be pushed.

At the rears, how often have you flushed the fluid? Sometimes a layer of crud will build up at the limits of the piston motion. When the pad wears then the seal ends up getting stuck on that ring for a bit, which causes the pads to ride against the disk.

Try taking the Air control valve off the servo and seeing if D is sticking, or P isn’t sealing. Also check that the servo filter isn’t clogged (and everything else that might cause an issue):

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3

All the pipes and pads have less then 2000 miles on them from new , the car was just as easy to push with the servo connected !
It’s only a 5 min job to reconnect the servo , so think I will do that , jack one side of the car up , and get the wife to start it up and press the brake pedal , as I try and spin the wheels , should know if the brakes are holding on too long !
Not checked the servo filter yet !

How hot do the hubs get , normal ?

4

One of the failure modes of the air control valve is that they don’t stick permanently, but just delay on release a bit. I’ve seen it happen that they release by the time you’ve crossed an intersection, which makes it devilishly difficult to notice - but it really stands out on a spirited bit of track use.

That depends hugely on what kind of braking was going on shortly before you stop the car to check them.

If you go for a highway run, find a long layby, change down through the gears and roll to a stop (i.e. air flowing to cool the discs/calipers, no brake use to stop the car) then the rears are usually marginally warmer than the fronts.

If there is a significant difference then you’ve got either a dragging brake, or your wheel bearings are dry. (you do follow the greasing maintenance schedule, including the door hinges, right?)

5

Like the brakes , wheel bearings have less then 2000 miles on them , but my winter job is going to be go round with a grease gun !
I did not drive fast yesterday , with no servo , used the brakes sparingly
Air control valve sounds like it is sticking , but only a few seconds , so don’t think that few seconds would build up heat too much , as the car is stopped , very few revolutions with it sticking , at low speed !

6

It doesn’t take much. You can get an avalanche effect where the fluid starts to warm up and starts to apply the brakes because it doesn’t have anywhere else to expand (This depends on the configuration of the master/servo - I don’t think the Servo on the Mark 2 is prone to this).

Do your greasing now. Winter is for wood refinishing.

7

Well I just jacked the car up , wheels are free , started the engine , wife operated the brake , a few times , brakes are not holding on at all , next test will be take the car for a long run , know just the road , will not have to use the brakes for a good 4 miles , then stop and feel the hubs !
Yesterday it was 29 and sunny , today its raining , that’s the UK for you lol

8

Well , just a update , just drove for around 12 miles , most of it I did not have to use the brakes , stopped and checked the wheels , they was all cold !
On the way home had to use the brakes a bit more , fronts cold , rear just a little warmer then cold !

I am thinking when I have to use the brakes a lot , with that few seconds of the servo is holding on , must build up the heat , maybe I should use the handbrake more at lights , not have my foot on the pedal , give that few seconds to let the servo release !

9

Have had this happen, Andrew. A 38 deg © day in Melbourne. Brake pedal rock hard on the freeway (after a slow drive through the city) and smoke from the rear brakes. I let things cool and released the master cylinder nuts about two turns which allowed fluid back to reservoir and kept driving to Sydney. I made a spacer for the after market master cylinder and no more problems. Paul

10

Interesting. I take it you mean a spacer under the mounting of the M/C? I think that would suggest that the end of travel on the after market Master is too short (i.e. the pedal is stopping it short of end of travel), and not exposing the return port to the reservoir… The only problem with that theory is that this would make it almost impossible the fill the system from the reservoir.

11

I have to admit all I did was enhance a “side of the road” fix. I have not really thought it through. At some stage I will get a better dimensioned M/C. For the moment it works and I don’t think there is any means of adjustment? Paul

12

Too answer question - yes - under the M/C. Paul