Possible brake master cylinder problem

[kitchenman]

Since changing the front pads I have noticed the pedal will go down slowly if I press really hard
The brakes seem good and even with new pads of low milage the Mav stops well
I suspect the seal on the master is letting by a bit
Is it fairly simple to recon or best to change for new
When I worked in a garage in the 60's it was something I did many times but things will have changed, I do not recall servos in those day.
If the engine is off the pedal goes solid and will not go any further it is only with the engine on that the pedal carries on slowly going down but I need to tread very hard.
The reservoir is filled to the mark
Any ideas out there? :bow:bow

 

[makeitfit]

Did the pistons pop out at all? Any chance of air in the system ? Failing that it could be the pads still settling in :thumbs

 

[kitchenman]

No the pistons were pushed back as suggested by Rick with a 'G' cramp, took quite a bit of force.
The pedal is not spongy so I am sure it is not air and as I said with engine off solid. So may be servo related?
The only odd thing was no increase in fluid level with new pads, with other motors I have owned new pads or linings usually meant a level rise.
I have had the Mav for 4 years is it time I changed the fluid?
When I got it I changed all the oils but did not do clutch steering or brakes, would it be a wise move, I have an auto bleed bottle, have never done power steering.
No doubt that stuff is either in a workshop or in the manual

 

[solarman216]

I have experienced the "no overflow of fluid when compressing the caliper pistons several times, just seems that the reservoir is big enough to accommodate it, like wise I am experiencing the sinking pedal with engine running on my off road motor, have changed the master 3 times and still the same, cannot believe all 4 masters are bad? am still looking at this one, have yet to strip a master cyl, but afraid many more, more urgent jobs call, hence no off roading for a while, what I did find was if changing just the master cyl, it can be difficult to get the O ring to seal as it swells and so does not seat in the recess on the servo sometimes but hard to see, result is no servo effort as vac is lost, Rick

 

[Ogri]

Both of my diesels have done this and I think it's quite normal :

http://www.brakesint.co.uk/technical.html#techNote202

 

[kitchenman]

Both of my diesels have done this and I think it's quite normal :

http://www.brakesint.co.uk/technical.html#techNote202

Well that is a relief if Rick does not have the answer and their is paperwork to say this is normal I will stop worrying.
Thanks again for all the input :clap:clap

 

[solarman216]

Sorry but I have to say that is a load of crap, why do none of my other road going vehicles exhibit this so called fault? what I am describing on my off road motor is with the engine running and foot on brake, as you would at lights for instance the pedal sinks to over half way very quickly, this can be done repeatedly with no sign of leaks and no fluid loss, this to me can only be attributed to master cyl failure, I can only assume that once a MC has been off the car that something collapses, as said have not had a chance to strip one yet, as for the quote that because diesels have a pump as opposed to the vac in a petrol, I really cannot see this making any difference, vac is vac and the pump type is slower to produce vac than the petrol at idle, Rick

 

[solarman216]

Well that is a relief if Rick does not have the answer and their is paperwork to say this is normal I will stop worrying.
Thanks again for all the input :clap:clap

Sorry but it cannot be normal or acceptable, Rick

 

[solarman216]

Just to clarify I am not including ABS brake systems here, that is a different scenario altogether, Rick

 

[Ogri]

Sorry but I have to say that is a load of crap, why do none of my other road going vehicles exhibit this so called fault? what I am describing on my off road motor is with the engine running and foot on brake, as you would at lights for instance the pedal sinks to over half way very quickly, this can be done repeatedly with no sign of leaks and no fluid loss, this to me can only be attributed to master cyl failure, I can only assume that once a MC has been off the car that something collapses, as said have not had a chance to strip one yet, as for the quote that because diesels have a pump as opposed to the vac in a petrol, I really cannot see this making any difference, vac is vac and the pump type is slower to produce vac than the petrol at idle, Rick

Rick, I have massive respect for your technical knowledge and practical ability but this does seem to be a known characteristic of some diesel vehicles. Google "diesel creep" and you get thousands of results reporting the same symptoms. The MOT handbook seems to recognise it and says that it's not a fail unless the pedal reaches the floor as long as the pedal remains firm with the engine off.

I'm very happy to be proved wrong and I try hard not to talk/type crap.

Regards, Ian.

 

[spautoparts]

Ive had this on a few terranos and other 4x4s. After changing the pads the pedal seems to creep down about half way or more. Even after fully bleeding the system it's still the same. They do go back to normal eventually.

 

[kitchenman]

Hi all, thanks for the further update, I have looked at the manual and page BR-16 says
CAUTION:
Do not disassemble master cylinder.

How much DOT 4 will be needed to refil the braking system after I drain it?

The CREEP only happens to my Mav when engine is on

 

[macabethiel]

Since changing the front pads I have noticed the pedal will go down slowly if I press really hard
The brakes seem good and even with new pads of low milage the Mav stops well
I suspect the seal on the master is letting by a bit
Is it fairly simple to recon or best to change for new
When I worked in a garage in the 60's it was something I did many times but things will have changed, I do not recall servos in those day.
If the engine is off the pedal goes solid and will not go any further it is only with the engine on that the pedal carries on slowly going down but I need to tread very hard.
The reservoir is filled to the mark
Any ideas out there? :bow:bow

All Mercedes Cars & ML etc do this. In fact if you have the stamina to hold the pedal down on virtually any Merc it will eventually reach the floor. It's if this happens whilst braking on the move you have a problem.
No one has given me an engineering / fluid dynamics explaination but it seems to have started when vehicle were fitted with pedal boosters instead of the old type servos. ABS systems awash with pumps and valves are part of the reason. You also have many braking systems with brake assist or an equivalent that sense panic braking somehow from pedal acceleration ?? With all these little extra valves and sensors it feels sometimes that when you press the pedal hard something is bending !
Sounds AOK and normal to me on a vehicle built post 1989.

 

[solarman216]

1st job for today, strip a master cylinder, report back later with pics, Rick

 

[solarman216]

stripping brake master

Master Cylinder ready to strip

 

[solarman216]

Reservoir off

 

[solarman216]

Rubbers off, the front brake one is straight forward, the rear is angled to fit onto the yellow tube that routes it to the rear of the rear brake rubber

 

[solarman216]

all the plungers out

 

[solarman216]

front brake plunger, you can see the disk behind the rubber, more on this later

 

[solarman216]

complete front set-up, left hand rubber does the work , the disk behind is a one way valve to allow new fluid in on the back stroke if needed (like when bleeding) it needs this disk set up as the plunger is a good fit in the bore and the disk is smaller in diameter, the right hand rubber separates the front from the rear