1967 C20 Master Cylinder Plumbing

[sodly]

Hi. I'm a 60-66 guy contemplating updating my single-circuit master cylinder to a dual-circuit in my bone stock drum/drum, non-power-brake 1966 C20. I'm told the 1967 master cylinder is basically a direct swap. However, when I look at pics of stock 1967 and 1968 C20s I notice that the brake lines coming out of the master cylinder go immediately into some sort of proportioning block which I didn't think was necessary with drum/drum brakes. Can anyone explain what's going on here? What's the block for? Were C10s plumbed this way too? Or is the block just a C20 thing? Thanks.

[jocko]

Sodly, all the dual res mc's had, at minimum, a distribution block that allowed for separation of the circuits should you happen to get a big leak on one side (i.e. in an emergency, you'd only lose front or back brakes, but not both) - in fact, this is really the only purpose they served in the early dual res mc years. These blocks also had a switch that indicated a single brake circuit failure to the driver with a dash light - it basically trips when there is sufficient pressure differential and isolates the system.

EDIT: Sodly - thanks for the follow-on question below. I need to clarify the above statement, the more I look at it, the more it's pretty confusing. Maybe this will make more sense: "the purpose of the new to 67" combo of a dual res mc and distrib block was to isolate front and rear brake system fluid to ensure a single leak couldn't drain the whole system (the purpose of the dual res mc) and to notify the driver when a leak was detected in one circuit (the purpose of the block in these early dual res mc's).

For the conversion you're considering, you don't have to have a distribution block (and you don't have a dash light for it anyway), so you can hook directly to the reservoirs for front and back circuits. The isolation just takes place automatically - but you, as the driver, don't get notified (of a failure). So remember to check your mc res levels every once in awhile.

Here's a link to one I did in my 66 a few years ago.

EDIT: One more thought - there's nothing wrong with including a distrib block - you could even rig it up to an improvised idiot light or even make a brake warning light in your 66. I used the tandem lock placeholder in my 66 as a parking brake indicator - but you could easily do the same concept with a distrib block.

[sodly]

Thanks, jocko. I read some of your earlier threads on these conversions on the 60-66 side. I thought the whole function of the dual-circuit master cylinder was to isolate the front and rear brake circuits from one another. In that case it seems the block is redundant. Or am I missing something here? Why would they have both? Just to be double safe?

Also, does anyone know if there's a difference between a C10 MC and a C20 (or an Impala or Corvette, for that matter)? Are they all the same? Or was a C20's bigger/different?

[jocko]

Originally Posted by sodly

Thanks, jocko. I read some of your earlier threads on these conversions on the 60-66 side. I thought the whole function of the dual-circuit master cylinder was to isolate the front and rear brake circuits from one another. In that case it seems the block is redundant. Or am I missing something here? Why would they have both? Just to be double safe?

Also, does anyone know if there's a difference between a C10 MC and a C20 (or an Impala or Corvette, for that matter)? Are they all the same? Or was a C20's bigger/different?

If plumbed directly to the mc res, the circuits are always isolated - but without the block you lose the warning light function. Concept is the same though. The switch inside the block is just a little shuttle valve that senses pressure differential. There are sometimes differences between C10/C20 mc's. For instance, in 71-72, the C20 front/rear reservoirs are reversed compared to a C10. I'm NOT sure if that is true with a 67 C10 vs C20 reservoir. Bottom line is to buy a 67 mc for a C20 so it is sized properly for a 3/4 ton - and then plumb it the same way a 67 C20 would have been (that's the answer I'd need to dig up is just how a 67 C20 mc is plumbed - again, not sure if it's reversed from a C10 like it is in the disk brake era starting in 71).

[jocko]

Originally Posted by sodly

Just to be double safe?

Actually - excellent point, and just to clarify, I should've worded my first sentence much better in my first post. The block itself doesn't physically provide the isolation, that occurs naturally by virtue of the separated reservoirs, the block is primarily to provide the means for notification (my first sentence above is more associated with the new addition of the dual res mc in 67, not "just" the block - sorry about that, I didn't state that very well). In later model mc's, the block can have more functions (such as a combo valve or proportioning valve). I added a little edit in my first post to clarify, the more I read it, the more I think I muddied the waters.. ha. Anyway, hopefully this helped a little bit.

[sodly]

Thanks again, jocko. The more I learn about this the more questions I have. I've learned that some of these blocks act as Residual Pressure Valves to maintain 10 lbs of pressure in the lines at all times. They say drum/drum setups require this 10 lb pressure for a couple of reasons I won't get into now. Some later MCs had these RPVs built in. Otherwise they needed to be added as an accessory "block" such as in the photos I posted. At least that's how I understand it.

So if all drum/drum systems need that 10 lb RPV, did the original single-circuit MC from 1966 have one? Because they didn't have the accessory block. Hmmm.

[jocko]

Ah. For some reason, I thought the blue truck you posted on the other thread was a C10... Ha. I have the SAME picture and was looking at it think it would really prove the point if only it were a 20.. It is indeed a C20 - and when you compare it to the pic of the 71 MC in post 31 of the link I sent - I'd have to agree, I don't believe they are reversed on drum/drum mc's either. The 71 and 72 have them reversed above the block, so it's pretty clear on that 67-68 ish C20s that it's not reversed - and this is as we expected, so it's nice when something isn't too much of a surprise. I think you're good to go with a 67 C20 MC and the front res to front brakes, etc.

As for the RPV, I thought they were only necessary on frame mounted MCs and that gravity kinda kept sufficient pressure in the system on a firewall mounted system. This is worth confirming though.