B20 Amazon Twin Circuit - Front Brakes Binding
 

The front brakes on my 1969 twin circuit Amazon are slightly binding with a loudish intermittent rubbing/squealing sound when in motion. Have replaced the discs and pads as these needed doing, but still having the same problem. More on one side than the other. Discs can get quite hot as a result. Thinking it could be a sticking calliper issue, but could be something else. Servo seems ok as I've pumped the pedal a few times then switched on the engine and the pedal then drops by 3/4 inch or so. Is there anything else apart from the callipers that could be the problem? The brakes seem fine otherwise. It's like the callipers are not releasing fully. They're not that old, so maybe strip down and check out the internals.....?

Hi
it could be that the brake hoses have started to collapse and and holding slight pressure

you could test this buy just cracking the bleed nipples
then close them then drive the car and don't touch the brakes

When you replaced the pads I expect that you would have had to press the pistons back into the caliper body to insert the new pads. If the pistons retracted into the caliper body smoothly it is unlikely that you have a problem with the calipers.

Since you replaced the disks, was the mating surface on the hub for the caliper cleaned off to bright metal? If the surface is not clean the disk could be sitting 'high' on the hub resulting in inadequate clearance between the disk and the outside pad. With a wheel off you can insert a feeler gauge between the inside and outside pads and the disk to determine if the retracted clearances are significantly different (lack of clearance on the outside surface indicating that the disk is not sitting correctly).

If the rubber flex lines are of unknown age, grumpydad's suggestion of internal swelling in the line preventing retraction of the pistons is a possibility. However, it would be unusual for both front brakes to develop a swelling problem in the flex lines at the same time. However, if your old pads were really worn that might have provided additional pad - disk clearance which masked the swelling problem.

Finally, is the brake pedal retracting to its normal position? If the brake pedal is not retracting sufficiently fluid pressure may not be dropping enough to permit complete retraction of the pads.

brake issue
 

Its either the flexi hose breaking down inside and not permitting the flowba
ck of fluid or its the servo . flexi hose replacement first as its a lot cheaper !

Message from the peanut gallery.

I can't speak to the Amazon disc brakes specifically, but based on my experience with an MGA, my guess is that your piston seals are old and hard and need replacement. So, a sticking caliper issue. On my MGA brakes, the piston seal and dust seal flex when the pistons move forward to brake, and when pressure is off the brake pedal, the seals flex backwards and pull the piston back. The brake pad motion is quite small.

If your problem is deteriorating brake lines, you can diagnose that by cracking the bleeder and letting a little fluid out. If that is sufficient to relieve the pressure so the pads retract, then the problem is probably in the lines - or at least upstream.

A third option might be that a relief "bypass" in the master cylinder is blocked.

Checked out the calipers and brake lines and these are ok. Also, replaced the brake fluid. Some evidence of rust at the brake warning valve. Front brakes still binding but not as much. The brake master cylinder seems ok so my guess is either a deteriorating brake warning valve or an issue with the brake servo which sometimes feels hard....

Not sure how you could check the rubber brake hoses, they can look just fine externally, but cause a problem internally which can result in your brakes binding. If the age of the hoses is unknown then replacing them would be wise.

Did you remove the warning valve while bleeding? It can get locked in a warning position which would prevent successful bleeding.

"deteriorating brake warning valve "

I assume that you mean the brake circuit balance warning switch? If so, did you bleed the brakes initially and if you did this using the pedal pump method, did you remove the warning switch contact? As noted by C1800, if not, the pedal pump method will cause the unbalance piston in the warning switch to move. If you restore pressure to both circuits the piston moves back to its balanced position; but, may jam the the operating pin on the warning switch. This normally results in the warning switch being on all the time; but, it is possible that the pin is preventing the balance piston from completely returning to normal. However, this should still result in pressure returning to normal . It is hard to see how a problem with the warning switch could cause a problem with the pressure not returning to rest when you release the brakes.

As noted by C1800, how did you check the brake hoses? Swelling of the internal liner cannot be determined by visual inspection of the outside hose jacket.

Finally, di you try inserting a feeler gauge between the inside and outside surfaces of the brake disk and the respective brake pads. That will indicate whether it is a particular pad / circuit that is causing the bind on whether both pads and circuits are the problem.

Well, the brake binding issue has now been resolved. However, I still have this squealing noise at lower speeds (particularly from cold) which is related to wheel speed rather than the engine/transmission. The car is running 5.5J x 15 Minilite reps with 165/80 R15 Vredestein tyres. It could be one of the wheel bearings (one has recently been replaced), or could it be tracking? Anyone else has a similar issue running these wider wheels? Cheers.

Basics. Front or back? Can you replicate it with wheels in the air? Does the squeal vary at different steering angles, i.e. straight ahead or some lock? Check that you don't have one of the rear axle check straps rubbing on the inside of a tyre. I've had that.