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.

[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]

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]

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.

[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.

[Stocker]

Originally Posted by FirstOwner69

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

I just figured that out.... I thought I was responding to the OP!

[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
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[MFCopenhagen]

Awesome! I have a starting point!
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[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.
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[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.

[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

[FirstOwner69]

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

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. I've personally only seen the 3/4 ton trucks with the master that buldges around the rear chamber.

I'm curious since I have all the parts to convert my 69 C20, but haven't tackled it yet.

[chipflyer]

The OP has a K20. So, which way was the 3/4 ton master to be connected?

Like the one in the link of livrats first photo, post #6. Shape of the master cylinder doesn't mean much anymore. Take a look at the link below from autozone, they show 3 different styles for a 71 K20. I personally bought a new master cylinder at advance several years back and we looked in 3 different boxes with the same part number and each box had 3 different styles in them. Kind of surprised me and the guy behind the counter.

autozone master cylinders

[chipflyer]

I don't think the link to Livrat's post does a very good job clarifying this... unless I'm missing it.

Just wanted to point out that starting at post #6 just about anything you might need to know about boosters, master cylinders, and even the brackets for 1/2 and 3/4 ton trucks can be found with pictures. I believe this same info is also in the FAQ section.
Hope this helps

Jeff

[FirstOwner69]

Originally Posted by chipflyer

Like the one in the link of livrats first photo, post #6. Shape of the master cylinder doesn't mean much anymore. Take a look at the link below from autozone, they show 3 different styles for a 71 K20. I personally bought a new master cylinder at advance several years back and we looked in 3 different boxes with the same part number and each box had 3 different styles in them. Kind of surprised me and the guy behind the counter.

autozone master cylinders

I'm not trying to argue with you. Just trying to answer the OP's question regarding which MC line goes where and to clarify this for myself.

Your posts and Livrats' posts in the other thread seem to indicate that the lines from the MC would be front-to-front or rear-to-front depending on if the truck is a 1/2 or 3/4 ton and that 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?

EDIT: I just looked at the Autozone link for 71 C20s. They show 4 MCs, but there appear to be 2 types. Two say for front discs (with the bulged rear chamber area). The other two say for front drums. Didn't know any 71s had front drums.