Brake booster question. 72 K10 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

suburban99 · 03-18-2018, 04:07 PM

Ok here's my question...I have a 6.0 LQ4 and Th350 in my 72...I have been driving it lots lately. I noticed that the brakes didn't seem to be as good as they were with the old 350. I changed the front pads and installed a new Raybestos master cylinder as the old one had 1 bale on the top and leaked fluid out the top. Brakes have been bled many times and master was bench bled. I even jacked up the front of the truck so master was level...
The old 350 idled around 850-900, but the LS idles around 600-650 could lack of vacuum cause less braking power (more effort on pedal)? The pedal is rock hard with 1-1.5 inches of travel and truck will stop. Booster has vacuum if I pull off the vacuum hose after engine is off.

So could it be the proportioning valve?
Or should I convert to hydro boost?
Or get a different dual diaphragm booster setup?

RichardJ · 03-18-2018, 05:14 PM

>> old 350 idled around 850-900, but the LS idles around 600-650<<

Why not check manifold vacuum with a vacuum gauge?

Check the booster. With engine off, pump brakes 3 or 4 times and hold pedal. Start engine and the pedal should drop as vacuum provides assist.

B. W. · 03-18-2018, 05:27 PM

If your engine is stock or has a mild cam it should produce enough vacuum.

Measure your vacuum, some say you need 18"hg to work properly but I think 14 or above is good.

If your vacuum reading is low you can temporarily bump up your idle to get better vacuum & test your brakes.

If there is enough vacuum then test your booster, with the key off apply moderate pressure to the pedal, crank the engine with the brake applied, the pedal should drop slightly when the engine starts. If it doesn't drop & you have a rock hard pedal your booster is bad.

(Richard must type faster than me, he beat me to it!)

suburban99 · 03-19-2018, 12:04 AM

Pumped brake a few times then started truck with foot on pedal..pedal went down as it should...will test vacuum tomorrow.

suburban99 · 04-28-2018, 01:55 PM

Broke down and read the factory service manual for bleeding the brakes...made this tool to hold proportioning valve button in while bleeding...we’ll see if it makes a difference when my helper gets back...k

03-18-2018, 04:07 PM	#1
suburban99 Registered User Join Date: May 2014 Location: Mission BC Posts: 452	Brake booster question. 72 K10 Ok here's my question...I have a 6.0 LQ4 and Th350 in my 72...I have been driving it lots lately. I noticed that the brakes didn't seem to be as good as they were with the old 350. I changed the front pads and installed a new Raybestos master cylinder as the old one had 1 bale on the top and leaked fluid out the top. Brakes have been bled many times and master was bench bled. I even jacked up the front of the truck so master was level... The old 350 idled around 850-900, but the LS idles around 600-650 could lack of vacuum cause less braking power (more effort on pedal)? The pedal is rock hard with 1-1.5 inches of travel and truck will stop. Booster has vacuum if I pull off the vacuum hose after engine is off. So could it be the proportioning valve? Or should I convert to hydro boost? Or get a different dual diaphragm booster setup? __________________ 1962 C10 Longbox LS 5.3 4L60E----Sold$$ 2014 Ram 4x4 3500 Laramie Diesel..Momma's truck 2015 Ram 4x4 1500 1997 Boxster. 1972 K10 Custom Deluxe LS 6.0 1950 GMC 3100 S10 & LS swapped

03-18-2018, 05:14 PM	#2
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,466	Re: Brake booster question. 72 K10 >> old 350 idled around 850-900, but the LS idles around 600-650<< Why not check manifold vacuum with a vacuum gauge? Check the booster. With engine off, pump brakes 3 or 4 times and hold pedal. Start engine and the pedal should drop as vacuum provides assist. __________________ '67 GMC 2500, 292, 4spd, AC

03-18-2018, 05:27 PM	#3
B. W. Registered User Join Date: Nov 2014 Location: Bigfork, Montana Posts: 1,137	Re: Brake booster question. 72 K10 If your engine is stock or has a mild cam it should produce enough vacuum. Measure your vacuum, some say you need 18"hg to work properly but I think 14 or above is good. If your vacuum reading is low you can temporarily bump up your idle to get better vacuum & test your brakes. If there is enough vacuum then test your booster, with the key off apply moderate pressure to the pedal, crank the engine with the brake applied, the pedal should drop slightly when the engine starts. If it doesn't drop & you have a rock hard pedal your booster is bad. (Richard must type faster than me, he beat me to it!)

03-19-2018, 12:04 AM	#4
suburban99 Registered User Join Date: May 2014 Location: Mission BC Posts: 452	Re: Brake booster question. 72 K10 Pumped brake a few times then started truck with foot on pedal..pedal went down as it should...will test vacuum tomorrow. __________________ 1962 C10 Longbox LS 5.3 4L60E----Sold$$ 2014 Ram 4x4 3500 Laramie Diesel..Momma's truck 2015 Ram 4x4 1500 1997 Boxster. 1972 K10 Custom Deluxe LS 6.0 1950 GMC 3100 S10 & LS swapped

04-28-2018, 01:55 PM	#5
suburban99 Registered User Join Date: May 2014 Location: Mission BC Posts: 452	Re: Brake booster question. 72 K10 Broke down and read the factory service manual for bleeding the brakes...made this tool to hold proportioning valve button in while bleeding...we’ll see if it makes a difference when my helper gets back...k Attached Images __________________ 1962 C10 Longbox LS 5.3 4L60E----Sold$$ 2014 Ram 4x4 3500 Laramie Diesel..Momma's truck 2015 Ram 4x4 1500 1997 Boxster. 1972 K10 Custom Deluxe LS 6.0 1950 GMC 3100 S10 & LS swapped

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