Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Brakes engaging when the car heats up

My 94 plus 8 disk brakes start dragging when the car heats up. You can feel it when you come to a stop, it grabs and doesn't roll when the pedal is released. I jacked up the car and confirmed it at the wheels as well.

I thought that it might be too much fluid in the master cylinder so I siphoned some out with no improvement.

Car has headers and dual exhaust fyi.

I am wondering if master cylinder that is located close the the headers could be causing the issue?

Anyone have experience with this type of Thing? It's a first for me. Thanks

Could be thermal expansion of the disc pistons. I might pull the wheels and pull the pads and pistons. Clean the pistons to be sure they aren't rusty or corroded in any way and free to move in and out properly. I have never heard of temp sensitivity with brake fluid.

Cheers,

Does the brake pedal have enough slack movement. If the pedal is too "tight" the breather hole in the master cylinder might not be able to vent and cause the effect of the brakes staying on.

I made this mistake when I changed my brake master cylinder.

I'll second Richard's advice. This is exactly what happens when the master cylinder cannot fully retract.
regards,
Phil

Are you using silicone brake fluid by any chance? I have had this issue with a TR3 and an Alfa after changing to silicone fluid. In both cases the brakes took a couple of seconds to release fully when hot after coming to a halt and releasing the pedal. They didn't remain hard on though, just sticky. I think there is possibly less lubricity in the silicone fluid and it took longer for the caller pistons to retract. Just a thought but could be barking up the wrong tree.

The advice from SimonH here was spot on when this happened to me.

A sticking servo can also cause momentary drag.

I felt some play in the linkage with my foot, but have not confirmed if there is adequate play in the right places on the linkage. I will do that.

Also the sticking is not momentary fyi. Thanks

Simple check would be to back it off a bit and go for a drive (after a few cold and slow checks the pedal still works OK).

Hi Bluesman, I had a discussion with Peter Mulberry, about the standard brake callipers on my 96 +8 when I had it. I had similar issues start but evident as a judder on light braking that only went when the pedal was pressed hard. Peter said that the issue with these callipers which should be the same as those on your 94 +8 are that they distort after many years and then the pistons can become sticky. In your case it sounds like you have one or two that are holding the piston out after braking.

I firstly would check that the disks on each side are running true and in tolerance, there should only be a few thousands of inch of variation. If more the disks could have overheated and distorted. To check true jack car up at front putting on axle stands, make sure it is safe and chock the rear wheels, and in gear. Remove one front wheel at a time and then get a metal object that will stand upright that you can clamp a suitable sharp nail to.Place the pointed end immediately next to the disk at right angles so it just touches the shiny outer edge. Slowly spin the disk and see it wavers a lot from the nail point or rubs it. As already said it should only fluctuate a few thousands. Do each side and each disk, if no perceivable wavering then it is likely its the callipers.

If the callipers are the next step, then you can try and take them apart and replace the piston seals, although this does not always work as the actual calliper casing can distort and grab the pistons. There are 2 alternatives, send them of to have reconditioned or bite the bullet and upgrade to new callipers and possibly vented disks at the same time. Which is what I ended up doing in the end.

I hope this helps, I would ring Peter at Mulfab first and explain you issues he is very helpful.