Help with brakes please?

[malcolm]

On longish, gentle braking, such as going down a long hill, the brake pedal slowly sinks to the floor. Foot off and press again, and it's hard as a rock.
The discs are new, and new greenstuff pads, fluid changed and system bled. The issue arose before the work on the brakes, so it is not a result of poor bleeding.
Most likely cause? Slave or master? And are kits available that are of suitable quality?
Help would be much appreciated.

[christopher storey]

Any loss of fluid, Malcolm - or transference between either reservoir ? And while the pedal is sinking, is there any loss of braking effort ? Or is braking effort preserved despite the long travel - to be clear, I mean before you re-pump ? Are all 4 wheels being braked to the same extent ? I think my first port of call would be to local MOT station who could test the braking effort on each wheel. I have to say that I think this is air trapped somewhere in the system, but as you know this can be really elusive to eliminate - perhaps vacuum bleeding might make a difference

[malcolm]

Thanks for the answer Christopher.
There's no loss of fluid.
Braking seems constant on all 4 wheels.
There is a little loss of braking power, but the main problem is that the pedal travels steadily towards the floor!
The brakes were all bled at the garage that fitted the new discs, but I can't blame them as the problem existed before they did the work. It started when I was touring in Germany, and going down a long windy hill - pedal gently going down until I quickly release and then press again when it becomes rock hard again

[mgcjag]

I would go the easy route first...air in system...many have problems bleeding....but its easy.....if you go the old school method. ....Steve

[christopher storey]

Malcolm : I also wonder about water in the system - this can lower the boiling point of brake fluid dramatically( and the most common time for boiling to present itself is on a long , not necessarily fast, descent , when the pads are continuously in contact with the discs and thus heat transfer from disc to caliper is at its maximum ) . When you changed discs/pads etc, was the system drained and refilled or was it just topped up and re-bled ?

[Series1 Stu]

Surely, if it was air then it would be evident all of the time?

My first thought was that the piston seals in the master cylinder have failed. The piston will move fluid initially but on sustained braking the fluid will seep past the seals, causing the piston to move further in to the cylinder. Lifting off the pedal reverses the effect. You wouldn't necessarily lose fluid in these circumstances.

Regards

[abowie]

Surely, if it was air then it would be evident all of the time?

My first thought was that the piston seals in the master cylinder have failed.
Regards

If you're definitely not losing or transferring fluid then that's what I reckon too.

Fluid is refluxing into the reservoir, then when you pump it it goes back into the cylinder.

This could be either the MC or the one on the booster.

So rebuild/replace both.

[malcolm]

Thanks for all the info. The system was drained and re-filled with new fluid. I'll try a re-bleed, and then consider the MC

[abowie]

OK. Let us know how you get on.

[lowact]

In my case (same symptoms) it was corroded cylinder bores, caused by car sitting for a long time allowing moisture to collect in the inverts. Master and slave. At the time renewal was cheaper than resleeving.

[malcolm]

From what everyone has kindly said, I'm thinking Master/Slave (should probably say deputy instead of slave! )
As my garage did the disc/pad change, I'll get them to do a re-bleed etc to rule that out at their cost, then try the MC route