I have had a similar experience when the parking brake is slightly holding the brake shoes off of the anchor pins. Either by partially seized or sticking cables or the parking brake cables adjusted too tight. When the brake drum is removed see if you have play between the bar and the brake shoe. If you cant move the bar between the brake shoes try loosening the adjustment on the cables to get clearance.

Paul

JC2+2 wrote:

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You need to put it up on a hoist or jack stands and have someone press slowly down on the pedal so you can check rotation of each wheel to see whats going on.

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 John, i have already done this and it is normal. Nothing unusual in that regard. Thanks.

long stroke wrote:

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Prefectca wrote:

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I have had a similar experience when the parking brake is slightly holding the brake shoes off of the anchor pins. Either by partially seized or sticking cables or the parking brake cables adjusted too tight. When the brake drum is removed see if you have play between the bar and the brake shoe. If you cant move the bar between the brake shoes try loosening the adjustment on the cables to get clearance.

Paul

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 Paul, i will try that next time i tackle this mystery. Thanks.

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 At this time i do know that my parking brake is properly adjusted but i will try checking that other matter you mentioned. Thanks.

Just like Mark suggested, my guess is that you have a periodic loss of front braking. The symptom is exactly what will happen without fronts, the rears will lock at low speed. 

Cameron suggested checking for collapsed or blocked flex lines. Good start there.

If not any of that, then perhaps the integral hold off valve in the Combination block is seized, sticking or blocked.

That front valve could be essentially "holding off" far too much.

 

 

if you do replace the Valve, these kind of tools are invaluable when bleeding with a combi valve. It can be a real PITA without holding the spool centered. 

I remember trying to bleed the brakes on a 80 Parisienne with this type of valve, after 2 days we actually gave up and took the car to a shop. Being young, we hadn't a clue.

https://www.jamcosuspension.com/products/sfID1/33/sfID2/80/sfID3/130/productID/1481

cdnpont wrote:

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Just like Mark suggested, my guess is that you have a periodic loss of front braking. The symptom is exactly what will happen without fronts, the rears will lock at low speed. 

Cameron suggested checking for collapsed or blocked flex lines. Good start there.

If not any of that, then perhaps the integral hold off valve in the Combination block is seized, sticking or blocked.

That front valve could be essentially "holding off" far too much.

 

 

if you do replace the Valve, these kind of tools are invaluable when bleeding with a combi valve. It can be a real PITA without holding the spool centered. 

I remember trying to bleed the brakes on a 80 Parisienne with this type of valve, after 2 days we actually gave up and took the car to a shop. Being young, we hadn't a clue.

https://www.jamcosuspension.com/products/sfID1/33/sfID2/80/sfID3/130/productID/1481

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 Mark, thank you very much for this information. I will be ordering that special tool for bleeding because i really suspect it is my combination valve that is acting up here. I am trying to locate an original GM valve because i refuse to put Asian garbage on my cars. Once i do that i will bleed with that special tool you mentioned here and see what happens. Thanks. 

 

Start spraying the line nuts now with good penetrating oil George from the master down. If never apart, don't be afraid to use heat on the line nuts when you attempt to remove them.

It makes all the difference between saving or ruining the line. I've always had surprisingly good luck separating the nuts and lines using heat. Of course unless severely rusted.

Could you pull the valve, gently disassemble and clean it yourself? If yes, I bet it's gummed up or stuck. Maybe someone will chime in if it's a DIY to take apart.

Sorry on the poor guidance ...make your own tool?

There is a ton of stuff on a Google Search on this.

 

long stroke wrote:

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cdnpont wrote:

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Just like Mark suggested, my guess is that you have a periodic loss of front braking. The symptom is exactly what will happen without fronts, the rears will lock at low speed. 

Cameron suggested checking for collapsed or blocked flex lines. Good start there.

If not any of that, then perhaps the integral hold off valve in the Combination block is seized, sticking or blocked.

That front valve could be essentially "holding off" far too much.

 

 

if you do replace the Valve, these kind of tools are invaluable when bleeding with a combi valve. It can be a real PITA without holding the spool centered. 

I remember trying to bleed the brakes on a 80 Parisienne with this type of valve, after 2 days we actually gave up and took the car to a shop. Being young, we hadn't a clue.

https://www.jamcosuspension.com/products/sfID1/33/sfID2/80/sfID3/130/productID/1481

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 Mark, thank you very much for this information. I will be ordering that special tool for bleeding because i really suspect it is my combination valve that is acting up here. I am trying to locate an original GM valve because i refuse to put Asian garbage on my cars. Once i do that i will bleed with that special tool you mentioned here and see what happens. Thanks. 

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 I had the same problem bleeding my son's 78 K10 truck. Once I got the prop valve centered there was no problem with bleeding or subsequent operation. 

With regard to John Stuart, I guess like many businesses they may be closed for a while. But they do a good job. 

By the way did you check with Classic Industries and some of the other Chevy parts suppliers as to whether they have a replacement for your valve?