I'm rebuilding my master cylinder with new seals. The seal that goes on the rear piston, at the end nearest the booster, is different from the others. It's part number 4270741, and has a solid ring and a thin flange. I don't know which way round it goes - is it with the thin flange towards the front? The diagrams of the MC in the workshop manual and Haynes manual seem to have multiple mistakes....
Thanks in advance for any help.
Master cylinder seals
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akershaw
- Posts: 23
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- Your car is a: 1977 Spider CS1
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18Fiatsandcounting
- Posts: 3798
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- Your car is a: 1969 and 1971 124 spiders
- Location: San Francisco Bay Area
Re: Master cylinder seals
I can't guarantee that this is the correct way, but I've always installed that rear seal so that the circular "channel" between the solid ring and the outer flange, faces toward the front of the car. In other words, the "free end" of that thin flange faces frontward.
My reasoning was that any brake fluid collected in that channel as the plungers moved in and out would push the thin flange seal tighter against the inner core of the master cylinder. Again, no guarantees that this is correct but it's always seemed to work OK.
-Bryan
My reasoning was that any brake fluid collected in that channel as the plungers moved in and out would push the thin flange seal tighter against the inner core of the master cylinder. Again, no guarantees that this is correct but it's always seemed to work OK.
-Bryan
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akershaw
- Posts: 23
- Joined: Tue Sep 10, 2013 5:05 pm
- Your car is a: 1977 Spider CS1
Re: Master cylinder seals
Makes sense. Many thanks, Bryan. I’ll go with this.
Never ceases to amaze me that I can post a query late on a Saturday in the UK and get a reply within a few hours from California - what a great forum this is!
Never ceases to amaze me that I can post a query late on a Saturday in the UK and get a reply within a few hours from California - what a great forum this is!
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DieselSpider
- Posts: 2130
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- Your car is a: 1978 124 Spider with Isuzu Turbo Diesel
Re: Master cylinder seals
Yes the open end of the cup faces the force of the fluid pushing back to keep it sealed tight against the pressure in a hydraulic brake system so away from the pedal in the master cylinder.
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manoa matt
- Posts: 3442
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- Your car is a: 1978 Fiat 124 Spider 1800
- Location: Honolulu, Hawaii
Re: Master cylinder seals
The rebuild kits that are available are for the OEM master cylinder design. It is typified by two removable bolts on the bottom of the MC that retain the pistons in their relative positions. Most of the aftermarket designs available today do not have the bolts on the bottom, rather removable inserts that are under the low pressure inlet tubes from the reservoir. The aftermarket MC's use different seals.
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akershaw
- Posts: 23
- Joined: Tue Sep 10, 2013 5:05 pm
- Your car is a: 1977 Spider CS1
Re: Master cylinder seals
Yes, I've been rebuilding one of the OEM MCs. I noticed that many of the new ones on the market were a slightly different design and didn't want to install a non-standard part - makes it so much more difficult to find the right spares later on.
The MC is now assembled. The seal I was asking about is quite tricky to fit, but you can hold the front end of the rear piston in a vice and stretch the seal aggressively to get it into its groove. The front end of the piston is narrower and runs inside the springs so isn't a critical machined surface. Fine to clamp it in a vice, in my opinion.
The MC is now assembled. The seal I was asking about is quite tricky to fit, but you can hold the front end of the rear piston in a vice and stretch the seal aggressively to get it into its groove. The front end of the piston is narrower and runs inside the springs so isn't a critical machined surface. Fine to clamp it in a vice, in my opinion.