If I hit the pedal hard and fast, pedal stops about halfway down and I get good brakes, however, if I push the pedal slowly it goes almost all the way to the floor.
Have replaced brake pads,
Brake lines are the aluminium ones,
No fluid leaks anywhere.
No evidence of any vacuum leaks,
Power booster is good and (no fluid inside).
Have bled the brakes every which way at least 5 times now,
Power bleed done today at Brake Maters shop.
I have disconnected ABS, no difference in braking.
This is the 2nd master cylinder, installed yesterday
It all started with brake fluid leaking from the reservoir mounting grommets, so I decided to replace the master cylinder, as it seemed more cost effective.
What could possibly be going on here?????

 

I would guess at bad seals in the master cylinder. they leak until you get to the 1/2 way point where the hard stop starts pushing on the second stage (rear brakes if I remember right) When you jam them on hard, the pressure makes the seals spread put more and fluid doesn't leak past them giving you brakes faster.

I'd be guessing a MC..

 

But I just replaced the master cylinder again yesterday, second replacement in 4 weeks, how could they both be bad?

 

For reasons unknown, non-OE master cylinders don't seem to work very well in these trucks. Many owners have reported on it. Strange, but true.

 

I replaced 2 MC in one year...

 

What was the brand of the last one ?

 

got my last 2 from Lucky8.

 

That's where I got mine as well.

 

Do you have a firm pedal before starting the Rover? Any change in the pedal after starting it? When started, does pumping the pedal have any effect? When bleeding all components (master cylinder and each wheel), are you sure that you were getting all, and I mean all, air out of the brake system?

Air is apparently entering the system at some point or points and the source(s) need to be identified. I’d recommend working backwards, one component at a time, from the master cylinder towards each of the wheel cylinders; thru the ABS pump. First step being to bench bleed the master cylinder to ensure no air is in this component. Be prepared to remove all the brake lines that service the wheel cylinders from the ABS pump and replace with block-off fittings. Test pedal. Cycle thru each service line reconnection by bleeding and checking the pedal.

We’ve experienced a wheel cylinder bleeding within itself (no evidence of leaking) that presented a soft pedal when the vehicle started. While not the symptom you describe, perhaps the aforementioned diagnosis process might be beneficial.

I’m not yet familiar with the intimate details of a Discovery 1 braking system yet, but we also used a Tech 2 scanner to purge air from the ABS pump on a GM (but had no effect on our issue). Not sure if this is could be an issue with your Rover. We’re going to attack our ‘96 Discovery 1 this weekend - has braking/ABS issue.

Good luck...

 

Yes, the pedal is fairly hard before starting, if I keep my foot on it, it does go down slowly....after starting it goes down fast, however, when driving, if I step on it fast the brakes are good, if I push it slowly it goes down way low, but I still have good brakes at the bottom of pedal travel. As another member posted, I am suspicious of this aftermarket master cylinder, perhaps the piston seal for the rear brakes leaks, explaining why if I hit it hard it goes past that point quickly and reaches the second (front) chamber. Brakes have been power bled also.