75 Tourer Brake Pedal goes to floor

[T-Cut]

Originally Posted by Owen

I am beginning to think that the handbrake warning light is a concidence and that maybe it does need looking at.

I'm sure you're right. There's no systematic connection between the hand brake mechanism and the main brakes. The hb warning light is simply a mechanical switch on the lever, with no implications for the hydraulic system at all.

[workaholic_ro]

Originally Posted by Owen

The handbrake warning light is now also staying on ...

I had the same problem after adjusting the hb mechanism.
The cause was the handle "sleeve" which was not staying in the proper position. Push down the handle and check the light if still on.

[Beerhunter39]

Have to say that I had a firm pedal before replacing discs and pads and it still feels exactly the same. Maybe it's just car specific.

[T-Cut]

Originally Posted by Beerhunter39

Have to say that I had a firm pedal before replacing discs and pads and it still feels exactly the same. Maybe it's just car specific.

No the Sinking Pedal Phenomenon (SPP) seems to be present across the range.
Did you do the standard SPP test?

With engine running and car stationary, press brake pedal hard and hold down. The pedal will slowly sink down if it has SPP.
Turn the engine off, pump the pedal to discharge the servo vacuum and repeat the test.
The pedal remains rock solid with no SPP.


That sinkin' feelin'

[sworks]

Don't know how I missed this post ?? This happens on lots of different cars but does seem to be more noticable on diesels. When testing the servo for an mot this shows up - there were some special notices from VOSA a few years back if i remember correctly. One car in particular is the Hyundai Matrix diesel, you fit front pads and the pedal goes to the floor - take the car up the road and the brakes are perfect. Some of the Jags are the same. Nothing to worry too much about, i think it is only generally noticed when pads are fitted as you wouldn't normally hold the pedal down for a consideral time (in my opinion)

[rovermech]

Hi all
going back to my original post at the beginning of this thread its only vehicles with ABS that you open the bleed nipple when pushing back the caliper piston, If you push the fluid back through the system it first as to go through the ABS modual before reaching the master cylinder and thats where the damage can be done. Proof of the pudding, I have just had a MGF in, the owner fitted his own brake pads and pushed the fluid back through the system - result it failed the mot on brake pedal slowly creeping down under pressure - repair - second hand ABS modual and the pedal was ok again
he had fliped a seal and fluid was leaking past it.
As said in my early post on ABS system vehicles I always clamp the flex brake hose and push the fluid out of the bleed nipple, never had a problem and done to many to count. you can push the fluid back through the system on none ABS system's with no problems
Hope this helps - 55 years in the trade and still learning

[T-Cut]

Originally Posted by rovermech

- - - - - As said in my early post on ABS system vehicles I always clamp the flex brake hose and push the fluid out of the bleed nipple, never had a problem and done to many to count. you can push the fluid back through the system on none ABS system's with no problems

That's an interesting point about ABS and the SPP. However, I haven't yet replaced my brake pads, they are the originals, even though the car's getting on for six years old. I observed the SPP after replacing the hydraulic fluid at four years. I used a vacuum pump to draw out the old fluid via the bleed nipples. When I tested the brake pedal it showed the SPP. The pistons on this vehicle (with ABS) have never been pushed back for a service - that I'm aware of anyway. I must say you've got me wondering whether I've ever done so during my various servicing sessions. However, the frequency this seems to come up and with different makes of car suggests it has to be something other than ABS seals surely?

[David Lawrence]

Originally Posted by sworks

This happens on lots of different cars but does seem to be more noticable on diesels. When testing the servo for an mot this shows up -

I mentioned the source of the vacuum on a diesel previously but it was dismissed. But in my experience the vacuum on my CDT car seems to exhaust rather quickly compared to what I remember on petrol cars. Isn't it just that after changing pads, and following the instructions for bleeding by exhausting the vacuum before pumping the pedal, it is just the vacuum returning that gives rise to the sinking feeling?

Mine stops about half way down after some considerable pressing force, but when first applied it only goes about one quarter down, and then sinks to about half way as the vacuum returns. Continually pumping the pedal, to check on this seems to cause the vacuum to exhaust also with the engine running.
My feeling is that either the vacuum reservoir is too small, or the pump doesn't suck hard enough. Compared to an old petrol Astra I once owned the servo assistance seems pretty weak.

[T-Cut]

The MGR workshop manual says that the brake pedal sinks slightly as the servo evacuates. I doubt that a mechanically pumped vacuum (diesel) and an induction generated vacuum (petrols) are significantly different in character or efficiency. I assume the servos themselves are the same, but if not, there may be small operational differences. However, it's not the evacuation stage that creates the SPP. The effect happens anytime after the servo is operational. You must do the SPP test properly as I described earlier. I don't believe this phenomenon is due to an under-sized servo or poor vacuum. I think it's a design feature more than anything. Flipping of the ABS module seals may be a factor, but there are too many examples with Rover/MG cars and other makes to make this a servicing generated fault.

If you think your servo assistance is weak, I think I'd check the condition of the calipers, pistons, etc. A binding caliper mechanism will feel like the servo isn't helping much. Also check all the pneumatic pipework from vacuum pump to servo for possible leaks.

[Owen]

I seem to have set the ball rolling again and it is interesting reading. As for the connection between the hb light and the main system. The user manual does mention that it can be a sign of low brake fluid level. I checked that and and also that the handle was going all the way down when released which it is. The main brake warning is coming on more often but goes out quickly so I still think something is still not quite right. Is it possibly wear on the handbrake shoes I wonder?