Does anyone know if abs pumps have a rear brake pressure limiting valve in them?. My 4x4 has always just scraped though the MOT due to low service brake efficiency, the fronts lock on the rollers without a problem but the rears only get to 200kgs. Prefect balance but only 200kgs I found out that the master cylinder has a valve in it that only allows pressure to the rear brakes after a certain pressure is applied to the fronts so I thought it's got to be that. Fitted a new master cylinder and it's exactly the same. I've solved the problem by fitting some 43mm rear calipers in place of the original 40mm ones the pedal travel is a little longer but still within limits and the effort is up to 250kgs which is an easy MOT pass. But I would love to know what's causing the problem.
- Thread starter Bornfree
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Ubique
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The ABS block is normally pass-through (i.e. master cylinder controls braking pressure) unless there is a need to release the pressure to allow the wheel to rotate again (Traction control systems use the brake boost pump in the ABS block to apply the brakes without hydraulic input). Have you checked the actuator rod length from the pedal to the brake booster? is there a vacuum leak?
Wildefalcon
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Is there a load apportionment valve? One of mine doesn't operate the rear brakes unless loaded up.
The one's without abs have a load valve. But not the abs one's
Cannot hear any vac leak. The booster is certainly working as I can lock the fronts on a dry road without massive pedal pressure if I turn off the abs. The chap who does my mot's said that it could be that they are set with light rear brakes from new and they possibly all only scrape through but because they are so rare he's never tested another.
Wildefalcon
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Put an IBC in the back and fill it with water. See if it passes then.
