Help Diagnose Brake Issue

[jocko]

Originally Posted by chipflyer

The front chamber could be for the front or rear brakes depending on weather or not its a half or 3/4 ton, they are opposite! The link below with Livrats excellent info will explain it all very well and should help you decide exactly what you need.
http://67-72chevytrucks.com/vboard/s...d.php?t=214559

Jeff

I am reasonably sure that firstowner is correct on this one.

Livrat's input on the linked thread is very helpful - but does not apply to the OP's question about which chamber the front or rear lines go to.

All MCs that I am aware of, 1/2 or 3/4 ton, have the front brake lines connected to the rear chamber of the MC and the rear brakes connected to the front chamber of the MC. Livrat's comments are all related to booster interchangeability and the differences between 1/2 ton and 3/4 ton booster mounting, etc. I didn't see any info on line placement to specific reservoirs. Did I miss it? But the dead giveaway, as firstowner pointed out, is the size of the reservoir - larger res for the disk half of the truck, and the larger res is always in the rear of the ones I have seen.

I'm a little more used to older systems, so I'm interested to hear a definitive answer on this.

[Stocker]

Originally Posted by FirstOwner69

The OP has a K20. So, which way was the 3/4 ton master to be connected? I don't think the link to Livrat's post does a very good job clarifying this... unless I'm missing it.

Originally Posted by chipflyer

Like the one in the link of livrats first photo, post #6.

Originally Posted by FirstOwner69

Just trying to answer the OP's question regarding which MC line goes where and to clarify this for myself.

....the guidance I gave in post 20, where I stated the rear chamber (the larger) serves the front disc brakes, could be incorrect. I said this based on the fact that this discussion has been about a 3/4 ton 4WD truck. Livrat's 6th post seems to show this in the 1st picture. So, do you argee that the 3/4 ton should be front to rear regardless of the appearance of the 3/4 ton MC?

Originally Posted by jocko

Livrat's input on the linked thread is very helpful - but does not apply to the OP's question about which chamber the front or rear lines go to.

.....I didn't see any info on line placement to specific reservoirs. Did I miss it? But the dead giveaway, as firstowner pointed out, is the size of the reservoir - larger res for the disk half of the truck, and the larger res is always in the rear of the ones I have seen.

If I may try to sum this up, regarding where the lines go on the OP's K20... It seems as though all of the above quotes are saying pretty much the same thing, though in different ways.

Livrat may not say how the lines are to be connected, but his 1st pic, post #6, shows how they are connected..... rear of the MC to front of the prop. valve, then to front brakes. It is hard to see b/c it's partially hidden behind the other line, which goes from the front of the MC to the rear of the prop. valve, then to the rear brakes.

The larger MC reservoir -- which would be the rear one in the OP's K20 -- does serve the front discs. In this case, it is front-to-rear.

[FirstOwner69]

Boy, I sure hope we haven't scared a new forum member away!!
BY THE WAY, WELCOME ABOARD MFCopenhagen!

I also still feel the master in the picture he posted is incorrect for discs as it's exactly like the one that has been on my 69 since new. Can anyone verify that?

[jocko]

Thanks for clarifying Mike, that helps. So, the main reason this thread grabbed my interest is because of the 3/4 ton comment (what I have). OP's is a 3/4 ton also - so I strolled out to the garage and double checked to make sure I'm not losing my mind - and yes, I think we all agree(?) - my mc front res line goes to the rear of the prop valve and the rear mc res line goes to the front of the prop valve - i.e. matches post 6 (of the linked thread) pic #1, which is labeled as 3/4 ton, correctly.

So, I agree with your last statement of your post - and did before, I think
Thanks for pointing out that chipflyer was just referring to the pic, I was reading the whole thread word-for-word wondering how he pulled that info out of the linked thread - but you just cleared it up for me, thanks.

Also, I think what was confusing me a bit was that on a 67 1/2 ton drum/drum mc (no prop) the rear res is for front brakes and vice versa (which is contrary to the 1/2 ton info in the linked thread). But the linked thread was focusing on disk/drum, so apparently different rule of thumb for a drum/drum setup. The 67 manual dual res mc was built such that the piston itself did some proportioning by uncovering the circuit for the front brakes (in the rear reservoir) first when you press the pedal.

[chipflyer]

I also still feel the master in the picture he posted is incorrect for discs as it's exactly like the one that has been on my 69 since new. Can anyone verify that?

You are right I agree with that and said that in my first response. The OP bought a truck that had a front disc conversion but whoever did it left the pre 71 distribution block and didn't add the prop valve.
I think I muddied the water by saying that the lines could be attached to the prop valve either way depending on if it were a 1/2 or 3/4 ton, which is correct but in this case it should be front of res to rear of prop which I think is what we all agree on and I was trying to show with a pic in post 6 of the other thread.

Jeff
Posted via Mobile Device

[chipflyer]

This photo from the 1971 service manual may also help untangle this mess.

[Livrat]

I'll chime in here and see if I can help out...

MFCopenhagen, from looking at the picture of your current setup I do know that will not work correctly with a disc brake system, it appears you have the original 69 drum brake master cylinder and a booster that I am not familiar with.

There has been some discussion about what is the correct plumbing of the lines from the MC to the proportioning valve.
On 3/4 ton disc brake setups the front line from the MC goes to the rear of the proportioning valve. (1/2 tons are opposite)
The Picture post #14 above is the correct MC,booster and plumbing.

Also discussed is the shapes of the different 3/4 ton MC's, this would be difficult as I have seen several different styles which I do believe could be aftermarket replacements, most I have see from the factory have the "bulge" in the rear. Post #7 bottom picture.
http://67-72chevytrucks.com/vboard/s...d.php?t=214559

The one Identifying factor for me on the differences between 1/2 ton and 3/4 ton MC's is the depth of the rod going into the piston in the MC.
See pic #8 below
http://67-72chevytrucks.com/vboard/s...d.php?t=214559

IMO the easiest thing to do would be obtain a complete 71/72 3/4 ton MC,booster,brackets, and proportioning valve with the correct MC to prop valve lines, And all the 71/72 K series (4X4) front brake lines from proportioning valve to front brake hoses, (these can come from a K10 or K20). One thing to keep in mind that 71 had a different sized fitting on the passenger side line to brake hose than 72.

Hope this explanation helps out and was not too confusing.
Oh... and by the way.. WELCOME to the Board!!

[jocko]

Originally Posted by chipflyer

This photo from the 1971 service manual may also help untangle this mess.

thanks for posting that chipflyer - that helps put it out there very simply. One dumb follow-up, and I think I know the answer, but want to confirm (i'm having "transferrence issues" between my 60-66 time and my 67-72 time... ) The lines that go from the rear of the prop valve go to the rear brakes (and the lines from the front of the prop valve go to the front brakes), REGARDLESS of 1/2 ton or 3/4 ton status, correct? (i.e. not talking abouy the short lines from MC to prop valve - I now have a good understanding of the 1/2 and 3/4 differences there thanks to all the helpful posts above). Still working getting past my drum/drum 66 (with a 67 mc for dual res drum/drum-ness) set-up.

Thanks much chip and livrat

[chipflyer]

Yes, you are correct Jocko. This photo should also be included in this thread as it kind of helps tie it all together.


Also MFcopenhagen, I was up at teeitups earlier and he does have a good used booster, bracket, master cylinder and prop valve assembly for sale at a good price that would be an easy swap and get you put back together correctly.

Jeff

[jocko]

all makes sense now, life is in balance again! Thanks chipflyer, yep, that is the last piece of the puzzle. Thanks very much for the great answers. MFcopenhagen, thanks for posting your question - was very helpful to me too.