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Model A Brake Master Cylinder Unit - Help needed

  • 61xlch
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11 Oct 2022 13:30 #46155 by 61xlch
I would like to ask if somebody could help with larger drawings of the brake cylinder assy..
I have an Owners Service Manual copy, but all copied drawings in this manual are too small and too bad quality, in order to see small details.

Any lead appriciated.
Thanks
Andreas

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  • RandyEma
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03 Oct 2022 15:34 #46107 by RandyEma
Hello . Make a Teflon booster cyl cup That might take away some of that problem
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  • mikespeed35
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01 Oct 2022 18:56 #46102 by mikespeed35
Replied by mikespeed35 on topic Model A Brake Master Cylinder Unit - Help needed
There is a small relief hole in the master cyl. bore that is suppost to bleed this pressure off so the brakes release. Is this plugged or not being uncovered when the pedal is released?
CORDially Mike

Mike Huffman
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  • 61xlch
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30 Sep 2022 14:06 #46091 by 61xlch
I know DOT5 is harder to bleed. Used it already on numerous other cars with success. What is puzzling me this time:
When I have a good pressure point at the pedal after numerous strokes, it feels quite hard and holds this pressure very good and the needed stroke is VERY short. It doenst feel springy or soft at all, like it would be with air in it.
Only after a longer time (some hours or days) it looses this pressure point completely.

What is also unusual: even after I let the pedal back off and don´t touch it anymore, there continues a pretty strong and constant stream of fluid out of any bleeder screw, when I open it again.
No idea, what could be wrong. Possibly something is installed in a wrong way...

This brake issue stops all progress with my car. I am close to put it away again for the next 3 years...

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  • mikespeed35
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30 Sep 2022 03:57 #46088 by mikespeed35
Replied by mikespeed35 on topic Model A Brake Master Cylinder Unit - Help needed
I vaccum bleed with good results.
CORDially Mike

Mike Huffman
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30 Sep 2022 02:31 #46086 by Joel
DOT 5 silicone fluid is great stuff because it does not attract moisture, and it doesn't harm paint. It is all I use. But it is more difficult to bleed because tiny little bubbles form in it very easily. It seems quite possible that you still have air in the system by your symptoms.
When bleeding DOT 5 you need to do everything very, very slow, or you will stir up air that gets into the fluid. If you are using a pressure bleeder, use very low pressure in it, if you are bleeding by pumping the pedal go very slowly. About 10 seconds at a steady pace to get from a fully released peddle to the floor. Then close the bleeder screw and take another 10 seconds to slowly raise the pedal. After you are done bleeding, just let it sit for a day or two for the tiny bubbles to gather together into big bubbles and bleed it again, very slowly. Repeat as needed.
Good luck.

Joel Nystrom
1929 Duesenberg Model J Murphy Convertible Coupe
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  • adamarehart
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29 Sep 2022 22:10 #46084 by adamarehart
Replied by adamarehart on topic Model A Brake Master Cylinder Unit - Help needed
It has been reamed and sharpened, and a new, larger piston and freshly produced rubbers have been installed . Once it reaches a significant pressure, it maintains the pressure and does not collapse.
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  • RandyEma
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07 Jun 2022 15:17 #45532 by RandyEma
First of all I have never had any luck with silicone brake fluid . And I have been making the small booster cyl cups out of teflon which seems to hold the lip much better
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  • 61xlch
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07 Jun 2022 06:44 #45531 by 61xlch
Hello Mike,
Its been reamed and honed and fitted with a new oversize piston and new made rubbers. After achieving a considerable pressure it holds the pressure and does not fall through.

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  • mikespeed35
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07 Jun 2022 04:15 #45530 by mikespeed35
Replied by mikespeed35 on topic Model A Brake Master Cylinder Unit - Help needed
Did you sleeve the master cylinder?
CORDially MIke

Mike Huffman

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  • 61xlch
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06 Jun 2022 20:11 #45527 by 61xlch
Thanks George.
Regarding your hints: I used DOT5 silicone fluid, all hoses are new, no leakage outside. I am pretty sure it's bleeded 100%.
Must be a problem inside the master cylinder.

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  • hessacres
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06 Jun 2022 18:47 #45526 by hessacres
Replied by hessacres on topic Model A Brake Master Cylinder Unit - Help needed
Bleeding is a major chore. It is much easier if you use a brake bleeder that you connect to your air compressor. You can find them on Ebay for around $20. If you are loosing pressure that fast you clearly have a leak somewhere., look for fluid loss. What fluid are you using, I used DOT 4 after considerable flushing everything with denatured alcohol( until I did no see any visuable contamination). I also used hi temperature (white) lock tight on some threads to get good and tight and it seems to hold up to brake fluid, especially at the hose connections. Did you replace the hoses, mine were cracked to the point they weeped fluid when I got my car. I started the bleeding process at the master cylinder bleeder, then the right rear, then left rear, then right front then left front. Then the high one on the firewall that goes to the brake pressure gauge. If you are using a hand pump bleeder it is considerably harder to near impossible. Lastly do you have the original bleeders on the car. Someone in the past put some newer style on my car and they did not seal correctly, but when I replaced with original style they worked much better. I still have to re-bleed the system every year or two some especially the right front wheel for some reason. The car looks great, but to keep that way you do need good brakes. They may be hydraulic, but they are not ABS, and that is a heavy car.
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  • 61xlch
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04 Jun 2022 18:21 #45509 by 61xlch
Who can assist with insights and recommendations regarding the brake cylinder unit?
I rebuilt it carefully and complete, but it doesnt operate as needed.
Even after numerous bleeding procedures and double checks I am not able to achieve a constant pressure at the pedal. With countless pedal moves down to the floor it builds up a pressure, but after sitting half an hour it needs to be pumped up again.
Any ideas and experiences appreciated.
Thanks
Andreas
Berlin/ Germany

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