Bleeding Brake Mystery.
Discussion
I thought about the seals but yes it is the right way round. I did take photos just to check
I think it it has PDWA. I sat think because it has a block that looks like a PDWA but it has no switch. I did wonder whether any possible PDWA failure modes could introduce air into the system but can't think of any. It would also have to be some type of double fault since the old master cylinder works.
Gilbert56 said:
I thought about the seals but yes it is the right way round. I did take photos just to check
I think it it has PDWA. I sat think because it has a block that looks like a PDWA but it has no switch. I did wonder whether any possible PDWA failure modes could introduce air into the system but can't think of any. It would also have to be some type of double fault since the old master cylinder works.
Bypass the PDWA by using a junction block then re-bleed your brakes.I think it it has PDWA. I sat think because it has a block that looks like a PDWA but it has no switch. I did wonder whether any possible PDWA failure modes could introduce air into the system but can't think of any. It would also have to be some type of double fault since the old master cylinder works.
N.
Gilbert56 said:
Reluctant to replace the PWDA because there is strong chance that one of the 5 brake pipe connectors won't come off easily and if I have to replace the pipe its a pig of a job.
Why do you think that could cause the problem on the new master cylinder but not the old one?
Listen to what you are being told It's old news.Why do you think that could cause the problem on the new master cylinder but not the old one?
Pete
Good luck
Like Gilbert I am having difficulty understanding why the PDWA would cause this problem; system bleeds fine with old master cylinder, won't bleed properly with new. I would think this must point towards the MC.
I too hate the PDWA and threw mine away and would advise others to do the same at a convenient time but if it's working (I.e not leaking and the shuttle is in the middle) I would leave alone.
Gilbert, are the brakes ok with the old MC now?
Phil
I too hate the PDWA and threw mine away and would advise others to do the same at a convenient time but if it's working (I.e not leaking and the shuttle is in the middle) I would leave alone.
Gilbert, are the brakes ok with the old MC now?
Phil
Brakes usable with the old master cylinder . i.e. it did pass the MOT. but sure they used to be better. No air comes out when bleeding so suspect the old pads as it did seem to improve the more I drove it.
Interesting point about the free play on the servo. I wasn't aware it was adjustable. Will have a look in the morning.
Interesting point about the free play on the servo. I wasn't aware it was adjustable. Will have a look in the morning.
The free play in the manuals is to be taken with a pinch of salt but you need to check the piston gap of the new with the old, if there's a difference adjust to match the old. Post what you've done as it would be interesting to know, I queried it when I fitted a new servo as it was wildly different.
I've had trouble with bleeding brakes where excess air keeps getting in the cause in my case was new bleed nipples, they were getting air drawn in around the threads, cured by putting some ptfe on the threads.
Another bleed method is to crack a bleed nipple slightly and let it drain very slowly overnight, obviously slow enough that the the M/C doesn't drain out.
Air bubbles in the MC could be caused by the MC pushrod not been the correct length not pushing the seals far enough in or are pushing to short for a complete stroke.
Air can also stay trapped in the M/C though less likely when bench bled, but gently tapping the M/C with a hammer releases the air locked bubbles.
Another bleed method is to crack a bleed nipple slightly and let it drain very slowly overnight, obviously slow enough that the the M/C doesn't drain out.
Air bubbles in the MC could be caused by the MC pushrod not been the correct length not pushing the seals far enough in or are pushing to short for a complete stroke.
Air can also stay trapped in the M/C though less likely when bench bled, but gently tapping the M/C with a hammer releases the air locked bubbles.
If you can get some sort of 'a pedal' with the new master cylinder, pump it up and block the pedal down with a length of wood (against the wheel or seat). Leave it over night. See how it is next morning... and report back.
Other pro-tips: Red brake grease or silicone grease around the nipple thread (not PTFE tape) prevents ingress of air there, especially when vacuum bleeding (which is by far the best way, pressure bleeding can 'flip' a m/c seal if the pressure is too high).
If bleeding with an assistant, don't open the nipple until there is pressure in the system ie only when (her) foot is already on the downward stroke, or build pressure with a number of rapid strokes, hold foot down and then crack the nipple open briefly. This achieves more force and faster flow (to dislodge air) than by one gentle stroke.
Other pro-tips: Red brake grease or silicone grease around the nipple thread (not PTFE tape) prevents ingress of air there, especially when vacuum bleeding (which is by far the best way, pressure bleeding can 'flip' a m/c seal if the pressure is too high).
If bleeding with an assistant, don't open the nipple until there is pressure in the system ie only when (her) foot is already on the downward stroke, or build pressure with a number of rapid strokes, hold foot down and then crack the nipple open briefly. This achieves more force and faster flow (to dislodge air) than by one gentle stroke.
Edited by TwinKam on Tuesday 13th December 17:07
spent most of last night coming up with a theory why incorrect pushrod adjustment could come up with the symptoms I'm having. Has myself totally convinced this would give me the answer.
Checked it today and the adjustment is fine. aaaaagh.
I've now had more than enough of brake fluid for this year. Will now give it a rest over Christmas and see if next year brings any new ideas. ( maybe if i filled it with rum sauce it might work better)
Checked it today and the adjustment is fine. aaaaagh.
I've now had more than enough of brake fluid for this year. Will now give it a rest over Christmas and see if next year brings any new ideas. ( maybe if i filled it with rum sauce it might work better
)Gilbert56 said:
spent most of last night coming up with a theory why incorrect pushrod adjustment could come up with the symptoms I'm having. Has myself totally convinced this would give me the answer.
Checked it today and the adjustment is fine. aaaaagh.
I've now had more than enough of brake fluid for this year. Will now give it a rest over Christmas and see if next year brings any new ideas. ( maybe if i filled it with rum sauce it might work better)
Were the two pistons identical in length and detail?Checked it today and the adjustment is fine. aaaaagh.
I've now had more than enough of brake fluid for this year. Will now give it a rest over Christmas and see if next year brings any new ideas. ( maybe if i filled it with rum sauce it might work better
)how about an air leak at the point where the new master cylinder is connected to the brake pipes. A fault in the screw thread on the master cylinder? its the only thing that could explain the presence of air with the new but not the old, if you have already eliminated the pistons. try daubing something like thick grease or sealant around the join to see if that eliminates the air problem. Maybe the fitting on the new cylinder thread is not exactly the same as the old? There are several types of flare on brake pipe fittings.
Couldn't resist another play today even though I should have been helping get ready for Christmas.
Took both master cylinders apart and compared the dimensions of the pistons etc. Both identical. Only difference I could see was the new master cylinder had new looking seals as would be expected.
Then assembled the new one again. Filled it with fluid on the bench with clear plastic pipes from the outlets back into the reservoir (under the fluid). Pumped the piston until all the air was out and the plastic pipes were full of clear fluid. So far so good and I was tempted to put it back on the car and try again. Then noticed that pumping the piston I could get air bubbles to come up into the main reservoir. No on every pump or even consistent with the length of pump but still fairly frequently. Carried on long enough to be clear it was more than just trapped air. This was one of the symptons I had noticed on the car which didn't happen on the old master cylinder.
I then swapped the tipping valves. Did the same again and this time once it was fully bled I could not get air to rise into the reservoir.
So the tipping valve is a possible suspect. Altrhough they look identical the only noticable difference is that the spring is a bit stronger on the new one.
Still not conclusive until :
1) I can repeat it a few times in case it is an assembly coincidence
2) I can prove it is not just masking another problem.
Anyway now definitely sick of brake fluid everywhere.
Merry Christmas and thanks for all the sugestions so far
Took both master cylinders apart and compared the dimensions of the pistons etc. Both identical. Only difference I could see was the new master cylinder had new looking seals as would be expected.
Then assembled the new one again. Filled it with fluid on the bench with clear plastic pipes from the outlets back into the reservoir (under the fluid). Pumped the piston until all the air was out and the plastic pipes were full of clear fluid. So far so good and I was tempted to put it back on the car and try again. Then noticed that pumping the piston I could get air bubbles to come up into the main reservoir. No on every pump or even consistent with the length of pump but still fairly frequently. Carried on long enough to be clear it was more than just trapped air. This was one of the symptons I had noticed on the car which didn't happen on the old master cylinder.
I then swapped the tipping valves. Did the same again and this time once it was fully bled I could not get air to rise into the reservoir.
So the tipping valve is a possible suspect. Altrhough they look identical the only noticable difference is that the spring is a bit stronger on the new one.
Still not conclusive until :
1) I can repeat it a few times in case it is an assembly coincidence
2) I can prove it is not just masking another problem.
Anyway now definitely sick of brake fluid everywhere.
Merry Christmas and thanks for all the sugestions so far
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