Brake engagement lowers after about 15 min. of driving.

[elliottS]

Just got my SeriesII car back from installation of a new left frame member (Old one cracked and broke) The brakes are not right, the engagement point of the pedal starting push the master piston lowers until the pedal is almost to the floor. No fluid level changes in the bottles. After some time of the car just sitting, the pedal starting to push the piston returns to a normal level. It will not pump up when it is in the lowered state. Maybe something with the vacume??

Any ideas.

Elliott S.

[JerryL770]

Sounds like a leaky master cylinder to me.

[abowie]

Failure of vacuum boost should make your pedal harder rather than softer.

To replace the L engine frame in (I assume) a LHD car they've probably removed the LHS brake caliper, the booster, some pipework and possibly the pedal box including the brake and clutch master. As a mechanic finishing a long job like this, the last thing you do, in this case bleeding and testing the brakes, might not get done with total thoroughness, leaving some air in the system.

Before you do anything else, thoroughly rebleed the brakes with particular attention to the caliper on the side of the new engine frame. Put a whole bottle of fluid through so you exchange all of the old fluid. If you can't do this yourself get the mechanic to do it.

Another possible cause (apart from air bubbles) is that there is some water contamination in the fluid. Water is incompressible as a liquid but once it boils as a vapour it might cause the low pedal. This would fit with the initial good pedal followed by soft pedal once you've driven it.

If the master cylinder is leaking the fluid usually ends up in the footwell, but it can first be absorbed by the trim material around the pedal box inside the car. It's worth looking at this area just to make sure that a leak is not actually your problem.