Technical Master brake cylinder

[Walt37South]

Hi, at the bottom of the master brake cylinder of my 1997 Ducato is a small screw. Thinking it was a bleeding screw I removed it, but I couldn’t screw it back in. The screw has a non threaded end of about 5mm. I removed the whole cylinder and took it apart. The only way to get this screw back in is to compress the first spring to the point where the part that looks like a small dumbell is behind the screw hole. Is this to limit the compression/decompression of the first spring? Put it all back together and started bleeding all brake cylinders by using the brake pedal (by my wife). No problem with rear brakes and no problem bleeding the bottom cylinders of front brakes either, but no brake fluid though the bleeding nipples of the top cylinders. Any suggestions of what I’m doing wrong?
Related question: Are the first 2 brake lines (looking from front of vehicle) on the master brake cylinder for the top cylinders of the front brakes? Any help much appreciated!

 

[ZaphodB]

You'll get the pipe arrangement from the diagrams here:

33102/01 - BRAKE HYDRAULIC SYSTEM

FIAT DUCATO '94 BENZINA (1994-2002)

eper.fiatforum.com

33102/00 - BRAKE HYDRAULIC SYSTEM

FIAT DUCATO '94 BENZINA (1994-2002)

eper.fiatforum.com

(I've guessed it's a petrol , but the diesel looks the same, but there appears to be two different looking master cylinders - just match the diagram to the one you have)

 

[bugsymike]

Hi, at the bottom of the master brake cylinder of my 1997 Ducato is a small screw. Thinking it was a bleeding screw I removed it, but I couldn’t screw it back in. The screw has a non threaded end of about 5mm. I removed the whole cylinder and took it apart. The only way to get this screw back in is to compress the first spring to the point where the part that looks like a small dumbell is behind the screw hole. Is this to limit the compression/decompression of the first spring? Put it all back together and started bleeding all brake cylinders by using the brake pedal (by my wife). No problem with rear brakes and no problem bleeding the bottom cylinders of front brakes either, but no brake fluid though the bleeding nipples of the top cylinders. Any suggestions of what I’m doing wrong?
Related question: Are the first 2 brake lines (looking from front of vehicle) on the master brake cylinder for the top cylinders of the front brakes? Any help much appreciated!

Bit wary of you taking brake master cylinder apart unless fully experienced in overhauling hydraulic brake cylinders, no disrespect intended.
Generally most modern brake master cylinders are Dual Circuit so two separate pistons inside the single cylinder bore. The idea is in normal circumstances they both work together, but in the event of a brake failure on one circuit like a blown brake hose etc. the idea is although you will get excessive extra brake travel , there should still be some braking effort on the other circuit.
You may have refitted that "stop screw" in the wrong position so affecting the travel of that second piston. Ideally a diagram of the workings of the master cylinder from the manufacturer would help, if available.
You often hear people say they had total brake failure , but in reality the brake pedal did not reach the floor although still frightening for them.
I did have a 1950s Land Rover where a brake pipe failed and on those older vehicles it was only a single brake circuit, so pedal did reach the floor, I ended up driving up a bank luckily with no damage.
On your vehicle if you are happy the brake master cylinder is correctly reassembled you could try to see if brake fluid comes out of those top bleeders by leaving the other bleeders open so that the pedal actuates that circuit easier whilst bleeding allowing full travel of both pistons in the master cylinder .
Incidentally I always bleed master cylinders at the pipes coming out of the master cylinder first, often if nothing else has been touched it can be all that is required if lucky.
Another thought is, if you have ABS brakes on your vehicle the fault could be there, generally not a big issue but sometimes it is necessary to use diagnostic electronics to bleed/activate ABS units during that operation, although after over 55 years in the trade I haven't had that problem on any I have worked on.

 

[ZaphodB]

I think once that screw was removed , you've messed up the master cylinder setup internally..

You'll either have to strip it down and check over the workings, see if there's another position for it , or get a replacement master cylinder.

 

[Walt37South]

Bit wary of you taking brake master cylinder apart unless fully experienced in overhauling hydraulic brake cylinders, no disrespect intended.
Generally most modern brake master cylinders are Dual Circuit so two separate pistons inside the single cylinder bore. The idea is in normal circumstances they both work together, but in the event of a brake failure on one circuit like a blown brake hose etc. the idea is although you will get excessive extra brake travel , there should still be some braking effort on the other circuit.
You may have refitted that "stop screw" in the wrong position so affecting the travel of that second piston. Ideally a diagram of the workings of the master cylinder from the manufacturer would help, if available.
You often hear people say they had total brake failure , but in reality the brake pedal did not reach the floor although still frightening for them.
I did have a 1950s Land Rover where a brake pipe failed and on those older vehicles it was only a single brake circuit, so pedal did reach the floor, I ended up driving up a bank luckily with no damage.
On your vehicle if you are happy the brake master cylinder is correctly reassembled you could try to see if brake fluid comes out of those top bleeders by leaving the other bleeders open so that the pedal actuates that circuit easier whilst bleeding allowing full travel of both pistons in the master cylinder .
Incidentally I always bleed master cylinders at the pipes coming out of the master cylinder first, often if nothing else has been touched it can be all that is required if lucky.
Another thought is, if you have ABS brakes on your vehicle the fault could be there, generally not a big issue but sometimes it is necessary to use diagnostic electronics to bleed/activate ABS units during that operation, although after over 55 years in the trade I haven't had that problem on any I have worked on.

Thank you for taking the time to reply so comprehensively. The only reason for taking the master cylinder apart was to determine in which position the pistons should be for the screw to be completely screwed in. The only position that I could find was as described in my original post. This would mean that the function of the screw (the unthreaded end) is to limit the stroke of the secondary piston. I am just trying to find confirmation of this. Does this make sense? I have searched internet for hours but couldn’t find any mention of the function of this screw. Taking apart the master cylinder (in workshop) is actually quite easy. It is just a matter of removing the retention circlip while compressing the springs. This allows pistons and seals to come out, of course noting the correct sequence for reassembly. I will try the bleeding of the top brake cylinders again, this time by suction with a small vacuum pump and I’ll follow your suggestion to bleed at the line connections on the master cylinder first. And no, my vehicle doesn’t have ABS. Thanks again.

 

[Walt37South]

I think once that screw was removed , you've messed up the master cylinder setup internally..

You'll either have to strip it down and check over the workings, see if there's another position for it , or get a replacement master cylinder.

Please see my original post and reply to bugsymike. Thanks.

 

[Walt37South]

You'll get the pipe arrangement from the diagrams here:

33102/01 - BRAKE HYDRAULIC SYSTEM

FIAT DUCATO '94 BENZINA (1994-2002)

eper.fiatforum.com

33102/00 - BRAKE HYDRAULIC SYSTEM

FIAT DUCATO '94 BENZINA (1994-2002)

eper.fiatforum.com

(I've guessed it's a petrol , but the diesel looks the same, but there appears to be two different looking master cylinders - just match the diagram to the one you have)

Thank you very much! Very useful!

 

[bugsymike]

Thank you for taking the time to reply so comprehensively. The only reason for taking the master cylinder apart was to determine in which position the pistons should be for the screw to be completely screwed in. The only position that I could find was as described in my original post. This would mean that the function of the screw (the unthreaded end) is to limit the stroke of the secondary piston. I am just trying to find confirmation of this. Does this make sense? I have searched internet for hours but couldn’t find any mention of the function of this screw. Taking apart the master cylinder (in workshop) is actually quite easy. It is just a matter of removing the retention circlip while compressing the springs. This allows pistons and seals to come out, of course noting the correct sequence for reassembly. I will try the bleeding of the top brake cylinders again, this time by suction with a small vacuum pump and I’ll follow your suggestion to bleed at the line connections on the master cylinder first. And no, my vehicle doesn’t have ABS. Thanks again.

Good luck.
I am not a great fan of vacuum bleeders, the reason being I believe on older/tired master cylinders the vacuum can cause air to be drawn past the back seals in master cylinder aggravating the problem. If a problem bleeding I use a pressure kit using air from a car spare tyre at under 20psi.
It is hard to say without being able to work on the master cylinder re the stop screw side.
Could you get a diagram from a different master cylinder that uses the same principle to give an idea of how it should function?