K5 Master cylinder differences?

[dyeager535]

Other thread on master cylinders got me to thinking about when I next change mine out.

Checked Rockauto.com, and an '83 K5 does not seem to share the same Delco part number that an '86 or later would for the K5. Both indicate an AL/plastic master.

I figured there might be a difference based on ABS, but '86 wasn't ABS, so that's not it.

Any idea what GM changed?

 

[TommyD11730]

Could it be single res vs dual?

 

[diesel4me]

Possibly they went to a "stepped bore" type vs a straight bore..?

The stepped bore ones have two different sized pistons for front vs rear..

 

[dyeager535]

Definitely not single vs dual reservoir.

For my purposes looking like the plastic/AL one will work better (doesn't rust) however looking into them, no idea which year I should choose. Probably need to look at a light 3/4 ton application since I have a 14SF rear.

Oddly, at least to me, is that while the '83 and '86 K5's list different masters, '83 and '86 K20 show the same one.

 

[diesel4me]

Maybe because 1/2 tons have smaller rear drums & caliper bores?..

Its always been tough to determine which brake system a GM truck has,once the options list dissapears...I hated having ask if it was a "JB5,"JB6",or "JB7" when I worked a parts counter,no one ever knew for sure....

I learned later on the JB5 is the lightest duty system as far as caliper,wheel cylinder and drum sizes (11"),while the other two were higher GVW and drum sizes (13")..GM also used 11-5/32" drums with slightly wider shoes on some 1/2 ton and light duty 3/4 tons,to add to the confusion..and had a number of various sized master cylinder bores..

Evidently its not real "critical" though,when a lot of rebuilders simply consolidate many different masters into one that was most common and they seem to work fine on multiple applications..

For the record I have seen several GM square bodies as late as '87 with cast iron master cylinders,that were known to be "all original" and some with the cast aluminum ones too...

 

[dyeager535]

Yes, but my understanding was that the K5's kept the same brakes across the board. I wasn't aware of any rear 10 bolt brake options that might drive master changes.