Land Rover Repair: 1998 discovery brake issue, wheel sensors, brake caliper

Question
Hi, I've got a 1998 discovery (the v8 petrol edition) and have a braking issue that is spardic. I've taken it to my local mechanic (a reasonable mechanic who we have found very good on every other issue) whose is having trouble sourcing the problem.

The problem seems to happen about once or twice a day and only at very slow braking speeds. I get a shudder, shudder feel in the pedal with very little stopping power. I then release the brake and pump it back on and its fine. The problem is a difficult one as I can just replicate the issue on call. We have bled the lines and replaced pads and rotors where needed. The ABS light does not come on during the shuddering. The vehicle has been on a bake testing machine (to check its stopping power) which shows its fine when the brakes are working.

When the event occurs it seems to feel that the brakes start to take some sort of grab to start the slow down but then very shortly afterwards start shuddering and no more stopping power than when the inital brake pedal was hit. It is fixed by one quick pump of the brake pedal (a release and 9 times out of 10 the next imediate push comes up firm, on the odd occasion its been 2 pumps).

Hoping for some direction
Regards JB (from NZ)

Answer
Hi James,

at first I thought of ABS wheel sensors or perhaps even the modulator but usually ABS light would trigger (unless the light is faulted too).  The next culprit I consider is the master cylinder.

How were the pads when you replaced them?  Did they seem glazed?  Do you use OEM or aftermarket parts?  IF its not the pads and your calipers are in good condition, you have to look at either the wheel sensors - are they fitted correctly or faulted?  

Macklow:

..."find sensor (it is a round tube at the top of the hub, not on the brake caliper), it has a black tube coming out from it that holds the wires that go back to the abs unit under the hood. You should only have two flexible tubey things leaving the brake area... the brake hydraulic line and the abs wire.

push down on the sensor until it stops moving. next time the disco gets driven, the sensors will be moved to have the proper gap. "

ABS modulator has been known to cause these faults.  You could try pulling the ABS fuse and going without ABS braking...to see if trouble goes along with ABS system.

Hope this helps.

JohnMc