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Master Cylinder Problems

Longknife

Well-Known Member
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12:12 PM
Joined
Jul 12, 2017
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Location
Illinois
Here is a story of 4 master four M cyls! I purchased my 69 RR in 2019. It had front disc conversion, single piston calipers, adjustable pressure valve for the rear, stock rear brake system, NOT power.
(1)The first cyl. leaked around the cap, as it was a rebuilt unit and apparently sand blasted on the top surface so I couldn't get it to seal. It was also bare cast and rusting badly. It seemed to work good though.
I replaced the unit with this master cylinder in the spring of 2021. https://www.classicindustries.com/product/all-years/plymouth/all-models/parts/mp500505.html.
(2) the second cylinder worked great but by late summer I could tell the pedal was getting low and require a pump to bring it back up to the top. It stopped great though. I bled and adjusted all the brakes, but the pedal continued to feel too low on the first stroke.
(3) In the spring of 2022 I purchase another M-cyl.
This cylinder look great and worked great too,, at first, but late in the year I could tell that the pedal was getting low on the first stroke and needed a second stroke to bring it back up. Bleeding and rear adjustments didn't help. This cylinder came with a 2 year warranty so I sent it back and received a new one.
(4) I installed the new cylinder in the spring of 23 and it brought the pedal back up and I had brakes again! BUT, by late summer the pedal began to drop too low on the first stroke and I had to give it a second pump to bring it back up to the top. The pedal felt firm but was just low like the two previous ones!!!
Here I am in 2024, and i need another Cyl. The first one replaced because of leaking around the cap but the last three were all replace because of the same symptoms!!!! I would like to identify exactly what is failing on these cylinders, and then I could diagnose, why is it failing? I have ideas but what are your thoughts??
 
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Quality of parts are poorer than poor anymore.
Mine was just the opposite. Master disc kit from Pirate Jacks.The master would stick and drag the brakes.
Took 2 to get it right.
 
Yep..... I agree with member Pnora, it says it all! The master cylinder bore is not true, it's called quality control. My buddy has his own machine shop at his premises, and he bored the master cyl and then put a sleeve in my master a few years ago. Can't remember the sizes he told me me, but it was way out of trueness. Works fine now.
 
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