Help Diagnose Brake Issue

[MFCopenhagen]

Brand new to the forum here...and brand new '69 owner (don't yet have a build thread).

I just bought the truck this week and need a quick list of things to check on my brakes.
Driving the truck home, I'd apply the brakes and it was like trying to stop on an icy road. I had to be really easy on the pedal. It's had a front disk conversion.

I haven't had really a chance to spend quality time checking everything out and I'm definitely not a brake guy.

Hoping to get some advice on some things to check and maybe some procedures to diagnose the problem. Thanks in advance!

[ChevyCruizer]

Trying to stop on an icy road...is it the front or the back that is locking up? I have other questions but will wait for a little more information before I type them all out.

[MFCopenhagen]

The backs are locking up.

I don't have a lot of options when applying the brakes...either VERY lightly applied, or start sliding down the road.

[BMERDOC]

Adjusted too tight? Fluid contamination of the linings? Pull the drums and let us know what you find.

[Stocker]

You have a '69 converted to front discs, and the rears are locking up. Make sure you have a (correct) proportioning valve. It's required b/c disc brakes need more line pressure to operate than drums.

Probably should have a vacuum booster also.

[FirstOwner69]

Originally Posted by Stocker

You have a '69 converted to front discs, and the rears are locking up. Make sure you have a (correct) proportioning valve. It's required b/c disc brakes need more line pressure to operate than drums.

Probably should have a vacuum booster also.

That's what I was thinking also. Post a picture of the master cylinder/booster setup and it should be easy to tell.

Edit: I see you have a K20. The 3/4 ton setup is different from the 1/2 ton... different master cylinder. Not sure of booster, but I think it's also different.

[Stocker]

Originally Posted by FirstOwner69

That's what I was thinking also. Post a picture of the master cylinder/booster setup and it should be easy to tell.

Edit: I see you have a K20. The 3/4 ton setup is different from the 1/2 ton... different master cylinder. Not sure of booster.

Doesn't matter if you're just trying to determine whether you have a vacuum booster. They will differ somewhat in appearance, but are basically the same -- a big round can between the firewall and the MC. Without one, the MC bolts to the firewall.

[FirstOwner69]

Originally Posted by Stocker

Doesn't matter if you're just trying to determine whether you have a vacuum booster. They will differ somewhat in appearance, but are basically the same -- a big round can between the firewall and the MC. Without one, the MC bolts to the firewall.

Agreed. My suggestion of posting a pic was meant for the original poster to show his setup. Didn't mean to mislead anyone.

[MFCopenhagen]

Starting the project after work tonight and will spend the weekend with it.

I'll take pics along the way and see what I can find.

[Stocker]

This is from a recent thread (not mine). It shows the MC bolted to a booster (big round can with a vacuum line attached), which is bolted to the firewall.


The proportioning valve is not visible in this pic.... it will be on the other side (drivers side) of the MC.

[MFCopenhagen]

I'll get a pic up once I get off work and get home.

--Really appreciate all the help with this.

[FirstOwner69]

Post #4 in this thread has a pic of the correct original GM setup for a 3/4 ton.

http://67-72chevytrucks.com/vboard/s...d.php?t=510104


Edit: ....and this is a picture of my original 69 3/4 ton setup with drum/drum brakes. This IS NOT what you want.

[71K20chevy]

This is the prop valve on my 71 K20, disc/drum:

[MFCopenhagen]

Just took this pic...what can you tell me from this?...

[chipflyer]

That's not a prop valve, that's the pre 71 drum brake distribution block. Your brakes won't work correctly until you get a prop valve as shown in the photos above.

Jeff
Posted via Mobile Device

[FirstOwner69]

Originally Posted by chipflyer

That's not a prop valve, that's the pre 71 drum brake distribution block. Your brakes won't work correctly until you get a prop valve as shown in the photos above.

Jeff
Posted via Mobile Device

Agreed. Plus your master cylinder is like the one I posted in post 13 that's on my 69 with drums. The correct master for 3/4 ton front discs is shown in post 14 (the rear chamber for the front brakes is larger).

[MFCopenhagen]

In another post I read that the front is larger.
Posted via Mobile Device

[FirstOwner69]

Originally Posted by MFCopenhagen

In another post I read that the front is larger.
Posted via Mobile Device

\

The chamber for the disc front brakes is larger. I understand, the front brakes should be connected to the rear master cylinder chamber (the larger one). On the other hand, the drum/drum trucks had the rear chamber connected to the rear and front to front.

[MFCopenhagen]

Awesome! I have a starting point!
Posted via Mobile Device

[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