P38A Random braking problem-sometimes extremely hard brake pedal

I have a problem with the brakes in my P38 4.6. 2000.
When driving, once in a few brakings, the brake pedal is very, very hard to press, so it's hard to brake. And a moment later it works normally.
When driving, sometimes (quite often) you can hear something like "trtrtrtrtr" near the brake pedal.

There are no errors on the dashboard. The lights are not flashing. There are no errors in the Nanocom. There are no leaks (at least I didn't notice anything with the brakes or the brake pressure pump).

For the test, I took out the black ABS relay. Of course, the errors appeared, but the brake worked normally for a long time (a dozen or so brakings), so I think the high-pressure tank is tight.

I don't have a RR repair shop far away, so I have to fix it myself. Looks like some kind of sensor? Any ideas where to start?

I would be very grateful for any advice!

sounds like the abs is activating when perhaps it shouldnt, i think this will give a hard pedal as it assumes you already have you foot on the pedal.
check the live data from the wheel sensors, could be a dodgy sensor or broken reluctor ring
how many pedal presses between the pump running?

Thank you for your answer.

Can I find live data from the wheel sensors in Nanocom? If a damaged ABS sensor at the wheel can cause such symptoms, maybe it would be worth disconnecting them one by one?

I don't know yet how many presses there are before the pump starts, I'll try to check (when I was driving without a relay, i.e. without a pump, it braked normally and effectively, I think about 20 times).


EDIT:
ABS sensor damage did not show an error on the dashboard?

Zaku said:

Thank you for your answer.

Can I find live data from the wheel sensors in Nanocom? If a damaged ABS sensor at the wheel can cause such symptoms, maybe it would be worth disconnecting them one by one?

I don't know yet how many presses there are before the pump starts, I'll try to check (when I was driving without a relay, i.e. without a pump, it braked normally and effectively, I think about 20 times).

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Live data is available on Nanocom. Disconnecting a sensor will throw a fault as the ECU expects pulses from the sensors.

Nanocom only collects liver wheel data during the diagnosis period, when the abs light is on. It goes out at about 5mph

kermit_rr said:

Nanocom only collects liver wheel data during the diagnosis period, when the abs light is on. It goes out at about 5mph

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That's definitely the case on Wabco-C, but Nanocom Wabco-D (1999 onwards) might be different ? The later guide does not mention it, but the earlier guide does ?

pwood999 said:

That's definitely the case on Wabco-C, but Nanocom Wabco-D (1999 onwards) might be different ? The later guide does not mention it, but the earlier guide does ?

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mine was D and worked this way