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bubbahotep 04-21-2013 09:40 AM

Master Cylinder Bore size, what size is this?
2 Attachment(s)
Is this master cylinder roughly 1 1/8" or is there no way to tell other than get inside the master cylinder? I included the piston because some info vaguely claims that the piston is the bore but thats not correct.

Ive got a car that had perfect braking prior to a few weeks ago when I swapped out the booster (dual 9" now) and master cylinder. Lines were cleared of air 3 times (all 4 corners), combination valve is functional, pedal radio is roughly 4:1 using the stock 86 gm hole, booster holds vacuum and car has a vac gauge. As it is now, the pedal is rock hard with barely a few inches of pedal push and I have to really struggle to stop the car. I cant get any brakes to lock up on front or rear. It feels like stopping a car with no vacuum assit (as an example) even though the booster is working.

New front disc pads from last year and new rear drum shoes from before the swap. The lack of stopping power seems to match anything related to the wrong bore size on the master cylinder. I had the cylinder for over a year and have no record of where I bought it from. A mistake I agree.

evolvo 04-21-2013 12:28 PM

Yes, the bore of the cylinder looks to be 1 1/8" from your first photo. The piece in the second photo, however is not the piston. You can see the piston in the first photo, it's located in the bore and has an indentation where the push rod goes. I also see what looks like elongated mounting holes with thread marks on the cylinder. Did it come this way? Also, IIRC, the pedal ratio should be closer to 6 to 1. That would definitely help with stopping.

bubbahotep 04-21-2013 12:42 PM

Thanks for the accurate info. That is the piston, however. I removed it from the master to get a size but you cant see the indent from the top view I guess.

And you are correct, it is a "used" master since Ive had it on the car for a week or so. The elongated holes I suppose would be for centering but it self centers. Any scuff marks are from me taking it on and off.

I got my pedal ratio based on: pedal top pivot to pedal / pedal top pivot to booster connection

Its the stock holes for the OEM 86 gm brake booster setup. Still think I should be looking for 6:1, its easy enough to make a new hole?

evolvo 04-21-2013 02:51 PM

So your saying the black piece in the second photo is the piston? But if the bore of the cylinder is 1 1/8 that black piece should be close, within thousands, to the same size as the bore. It's barely 5/8", doesn't make sense. In the first photo the black ring is a snap ring that holds the piston in the bore, if you remove that you'll be able to get the piston out, but you probably don't need to.
As far as pedal ratio. Say the distance from pedal pivot to push rod attachment is 3", and from pedal pivot to foot pad is 12", that would be 4:1. But you can't just re-drill the pedal arm to move the push rod attachment as that will change the arc of travel of the push rod and may put it in a bind, or push it over center. The best thing would be to lengthen the pedal arm to 18", but I can imagine the headaches that will cause, or change the pivot attachment of the pedal arm, even more trouble!
Reading back over your OP, maybe time to check that everything you swapped out is the correct stuff. :spank:

bubbahotep 04-21-2013 04:22 PM

Maybe my term is wrong. That's the little metal pin that the brake booster pushes into. It slides out easily of the master cylinder. It's not a piston, sorry

To change my ratio I can easily use a bracket to tilt the booster down, which brings the link arm up. Originally I had it on the top hole of my pedal (there are 2) and that's about 6:1 but only made my pedal force slightly easier.

Bryan59EC 04-22-2013 05:26 AM


Originally Posted by bubbahotep (Post 1668999)
Maybe my term is wrong. That's the little metal pin that the brake booster pushes into. It slides out easily of the master cylinder. It's not a piston, sorry

This piece is included in some master cylinders so that the same master can be used for power or manual brakes.

Manual brakes have a deeper bore in the piston to retain the brake pushrod.
Power brakes usually have a very shallow bore as the piston pin is retained by the booster.

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