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Engine struggling after heavy brake use?

sokoloka

sokoloka

1/2 ton status
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Working my way through the "to-do" list for my '73 K10 and this one's been puzzling me...

EVERY time I have prolonged brake usage (ride the brakes) whenever I let off the 350 stumbles over itself and stalls if I give it gas immediately. This has happened since I bought the truck a few months back. Stock setup, disc in front, drum in back, vacuum booster. Doesn't matter if I'm going uphill or downhill. Most prevalent when performing a "rolling" stop.

If I let it idle for a few seconds after letting off the brake, she'll usually come back to life.

I believe the engine has been cammed but have no specs.

What could be the cause? Possible vacuum leak somewhere?

I appreciate any and all help!
 
hmmm, was gunna add something but I really don't know
earmarking for later when there's a real suggestion :D
 
sounds like the booster is leaking to me.
 
shady said:
sounds like the booster is leaking to me.
Click to expand...
How do I go about confirming this vs a leaky vacuum line?

454Sub said:
Perfect time for hydroboost.
Click to expand...
Pulled one from the Pick-a-Part a few weeks back! Need to figure out how to test that it's still good and find a good how-to on putting a system together.

You ever get yours taken care of?
 
a vaccum tester is what I used. I just pumped it up to about -20 and watched. if it stays at -20 and doesnt drift towards 0 it should be fine.
 
I would also check the idle mixture as it may not be right. I don't believe this would be the sole problem though, but it may be partly to blame as the mixture may be thrown too far off when the booster is actuated.
Just something to consider.
 
6872xtc said:
I would also check the idle mixture as it may not be right. I don't believe this would be the sole problem though, but it may be partly to blame as the mixture may be thrown too far off when the booster is actuated.
Click to expand...
The truck runs RICH as hell with a very high idle... so that very well could be a factor too. Haven't figured out whether the PO had no idea how to tune his carb or if he raised the idle to keep the engine alive when it stumbles.

Add another to the list!

Question on vacuum / PCV lines. Does it matter whether the vacuum to the booster comes from the front or the back of the carb? (Edelbrock carb) The way everything is set up now the PO had the passenger side valve cover upside down and backwards to get the PCV to the front and the booster to the back. Want to get new vacuum lines just to be safe and have PCV going from the back and booster to the front?
 
thats the way my sons 72 is. booster is at back of carb. pcv front
 
shady said:
thats the way my sons 72 is. booster is at back of carb. pcv front
Click to expand...
Hm. I'll get some pics of my setup today and get them posted.

Thinking of swapping the master cylinder at the same time. What bore size should I be working with? 1.00" or 1.25"?
 
Just get the same one you got now as far as master cylinders go. Pvc goes in the front, and booster goes in the back. Also the booster should have a one way check valve in line between the carb and booster. Its a soda can shaped object slightly larger than a D cell battery.

Not sure of your carb. But if you use the all mighty Google you should be able to find how it was supposed to be set up from the factory. Ie how many turns out from fully seated the mixture screws are supposed to be set at. Then you can set it to stock, and tune from there.

Edit. On the Edlebrock, Im not perfectly sure, but about 90% sure the mixture screws, out of the box, are supposed to be set at 1.5 turns out from fully seated. Edlebrocks also have problems with sagging or incorrectly set floats, and thats a PITA as well. And when I set floats at the level they recommend, which I believe was hanging down 1 1/16in, it ran all sorts of ghey, I am pretty sure I set them at 7/8in and it was much better.
 
454Sub said:
Just get the same one you got now as far as master cylinders go. Pvc goes in the front, and booster goes in the back. Also the booster should have a one way check valve in line between the carb and booster. Its a soda can shaped object slightly larger than a D cell battery.

Not sure of your carb. But if you use the all mighty Google you should be able to find how it was supposed to be set up from the factory. Ie how many turns out from fully seated the mixture screws are supposed to be set at. Then you can set it to stock, and tune from there.

Edit. On the Edlebrock, Im not perfectly sure, but about 90% sure the mixture screws, out of the box, are supposed to be set at 1.5 turns out from fully seated. Edlebrocks also have problems with sagging or incorrectly set floats, and thats a PITA as well. And when I set floats at the level they recommend, which I believe was hanging down 1 1/16in, it ran all sorts of ghey, I am pretty sure I set them at 7/8in and it was much better.
Click to expand...

Going to go play with the idle mixture screws right now. DEFO don't have that aforementioned check valve. Time to see if any local part stores have one in stock.

Any way of measuring what bore MC I have without taking it off the truck yet? Still want to keep it driving until I get the replacement parts.
 
Not sure how they measure. I ended up buying one for a newer style 3/4 ton. I hate that stupid metal one with that wire clamping the cap down, PITA in my mind. So I went with the same sized plastic version, and as a bonus I can see the fluid level without opening it.

Its a pretty universal valve, I see them at Autozone when I peruse through there. Really, if it is the same size as the factory vac line, and its one way only style valve, it will work.
 
The primary check valve for the booster is the fitting that goes into the booster at the rubber grommet. I have seen and driven lots of trucks that don't have the other one and all is well, but verify that the check valve is working. You can do this by pushing on the brake pedal after you shut the engine off. If you hear air suction when the pedal is being pushed and after a few strokes of the pedal the brake feel gets harder, then the check is holding, IF THE BOOSTER IS WORKING. Some boosters will hold vacuum when the pedal is at rest even if the booster is bad. You could take the master off of the booster with the lines connected to measure the bore, or maybe a website like Rockauto will tell you.

As for the carb mixture screws, you should be able to start with them 2-2.5 turns out and then adjust for the highest vacuum level on a gauge, or the highest idle rpm's. But it sounds like your carb may have the idle speed adjusted high enough to be off of the idle circuit of the carb. Try turning the speed down to what you would consider a normal rpm if the mixture screws don't seem to do anything. Just remember or write down what you do with adjustments so if you need to go back to point "A" you can do that. I can't give you the measurement for the float level that I use as I have never measured it. Sorry. I do that with the carb top inverted.
 
454Sub said:
Also the booster should have a one way check valve in line between the carb and booster. Its a soda can shaped object slightly larger than a D cell battery.
Click to expand...
The "D- cell" is a filter:
chevy_nova_1979_power_brake_filter_oem_4397925.jpg


The check valve mounts into the booster:

mqdefault.jpg
 
I agree with the "probably a leaking booster" suggestions.

However, if the booster isn't leaking, I'd hook the gauge to the vacuum line with the truck running and see what kind of vacuum it's making. You mentioned it has a cam. Some aggressive cams have a nasty lope sound at idle due to overlap of the intake and exhaust lobes. This will also reduce manifold vacuum substantially. This is probably not the case, but if it is, it's the wrong cam for your truck. Those cams are for high-rpm engines - not trucks.

I thought I'd mention it since nobody else did. Again more than likely, it's a leaking booster. If air is leaking in through the booster (losing vacuum), it will affect the ability of the engine to pull air through the carb and hence fuel through the venturi. This will result in a lean condition and could cause a high idle and stalling. Bottom line, I'd check/fix booster and any other vacuum leaks before retuning the carb.
 
sokoloka said:
How do I go about confirming this vs a leaky vacuum line?
Click to expand...

Can't remember the numbers verbatim, but they are unimportant. Start truck, let idle for a few seconds. Turn truck off. All without touching the brake.

Let it sit about 45 minutes. Should have two full pedal assists before it fades. If you do, the booster/check valve should be good.
 
Thanks for the advice everyone! Took a few hours this weekend to swap in a brand new TuffStuff master cylinder / booster.

Gave the whole system a good flush and bleed. Replaced a rear wheel cylinder. Adjusted the rear drums.

Braking definitely is better, and the engine no longer stumbles after heavy braking, but I'm not really blown away by overall brake performance.

I have a Dana 44 on the front of my K10. Any replacement calipers out there I can bolt up (maybe something with two pistons?) to increase my stopping power?

Stainless steel flex lines from ORD are also next on the list in the quest for better braking.
 
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