Thread: Puma brake pedal has that sinking feeling...

Originally Posted by Samblers

Hi Y'all,

Its been a busy time for the old Defender (2010 110), in preparation for a mid-year trip. This weekend I changed the brake pads and discs on all 4 corners, which required the removal of all 4 calipers and a brake system bleed (which I did twice).

With the engine OFF, the brake pedal is really nice and firm; cannot be pushed to floor.

With the engine ON, the brake pedal is less wonderfully firm, and will slowly sink to the floor if foot pressure is maintained.

Is that normal?

I suspect not, and honestly cannot now recall 'normal' pedal behaviour from before the time that I did the work on the brakes.

Is something up with the vacuum system?

Any help with test, diagnosis and remedy suggestions much appreciated.

Cheers, Sam

Either air in the lines still(less likely) or a faulty master cylinder(more likely). The additional boost generated when the engine is on allows the master cylinder to have fluid leak past the piston. Not likely to be the booster I would think.

Regards,
Tote

It definitely feels like fluid bypassing within the master cylinder... but I can press the pedal really hard with the engine off and it’s rock solid.

What changes when the engine is on to make fluid bypass within the cylinder? Vacuum boost is present, yes, but shouldnt this only affect the pedal effort required, not the hydraulic side?

Cheers

This is puzzling.

Some more diagnostic info:

- I took it for a drive, did some hard braking on loose gravel to get the ABS working (it seems work - vehicle stops well and pulses felt through pedal as expected).

- Checked that I hadn't dislodged any ABS wheel sensors during my work. I hadn't needed to disturb them, but they seem all good.

- Removed both ABS fuses and tried the brakes - same symptoms. Engine OFF, Pedal Hard - Engine ON, Pedal slowly sinks.

It seems to be an ignition switchable hydraulic bypass/recirc. At one point, I pushed the brake pedal with the ignition off, it firmed up, then I switched Ignition on (WITHOUT starting the engine), and the pedal sank slowly to the floor again.

I don't think it can be air in the system - with the Engine ON I can keep pumping the pedal again and again, and it will keep slowly sinking to the floor.

???

Hi Sam
Did you check behind the master cylinder boot for fluid leak? From what you say it should be dry I guess.
I recall reading that is a special procedure for flushing the brake fluid, that involves use of a scan tool, to flush out the ABS unit. Wondering if air got in there?
Good luck with it.

Thanks Brid (and Tote)

No fluid leakage and car drives/ stops fine. Its possible that pedal behaviour has been like this for a while, its just that I was doing brake system work and have now noticed it.

I've ordered a OEM master cylinder and will see if that solves, since m/c internally recirculating when boosted seems to be the most likely explanation.

If that doesn't solve, i'll suspect the ABS unit and prob send it to a specialist... or just live with it. Hmm.

Also i'll consider changing my work practices in future - for e.g not pushing the caliper piston back into the caliper before opening the bleed screw first, in case this has forced crap back up into the m/c, as one old post suggested. Although, wouldn't the crap be going into the ABS unit first...?

Cheers, Sam

*Before you start the engine, press the brake pedal with normal brake pressure and hold.
*Start the engine, the pedal should drop say 1/3, if you continue to press the brake pedal harder it will continue to drop about 2/3.
*This is normal for any vehicle.
*Keep in mind, when you brake and have come to a full stop you are not ment to continue to drive the brake pedal through the firewall.

*Providing you havent used the cheapest softest brake pads, if the pedal doesnt feel spongy then it's all normal.

*When changing front brake pads and rotors on a late model Defender (2007 - 2016) you dont need to disconnect the brake lines, just remove the retaining clip on top of the swivel housing. Then carefully move the caliper and the free hard brake line to move it on top of the diff housing locating arm, I normally use a ocky strap to hold it in place so the caliper doesnt drop and break off the brake line.

Older Defenders have 2 retaining nuts on the bracket a top of the swivel housing, which can be modded so its a slot rather than a hole for later brake work.

P.S if you are that concerned, at some point during normal working hrs, call in and I can see what the pedal feels like.

Originally Posted by DazzaTD5

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P.S if you are that concerned, at some point during normal working hrs, call in and I can see what the pedal feels like.

Thanks Dazza that’s kind of you. I’ve now received the new m/c and will install it to see what changes, if anything... and if it still does same then will drop by to give you a look.

Modest pressure during driving stops the car fine and the pedal stays put, it’s only when applying firmer pressure that it is able to be pushed to the floor.

Cheers, Sam

Originally Posted by Samblers

It definitely feels like fluid bypassing within the master cylinder... but I can press the pedal really hard with the engine off and it’s rock solid.

What changes when the engine is on to make fluid bypass within the cylinder? Vacuum boost is present, yes, but shouldnt this only affect the pedal effort required, not the hydraulic side?

Cheers

The vacuum boost assist does just that, assist. it applies more pressure than you can easily do. Therefore it will overcome a weak seal that you probably cannot.

From your description that is probably not happening. Go and see Dazza and set your mind at ease.