68 c20 Brake Booster Hiss - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ChicoSmith · 05-01-2009, 02:41 PM

I have a leak near the rubber grommet where the plastic check valve inserts from the vacuum. I noticed fluid missing in the master cylinder reservoir closest to the cab and no pressure in the pedal during braking.

Any words of wisdom from those who've dealt with this problem in the past.

sleepertruck72 · 05-01-2009, 03:00 PM

My 72 C20 had the same problem with the reservoir closest to the firewall. I changed my rear wheel cylinders and I dont have that problem anymore. As for the check valve, If its not squared up against the booster it will hiss. Try replacing the vacuum line and leave it long enough to where the check valve sits flat against the booster.I would probly replace the check valve while youre at it.

**nightmare** · 05-01-2009, 03:50 PM

you may also have a leak behind the master cylinder. do you see fluid on the booster under the master? if you do, your master cylinder is bad, and you need to replace it.

ChicoSmith · 05-01-2009, 04:11 PM

Thanks guys.

Replaced the valve at lunch today and filled up the master cylinder, took it for a tester and it was a little better. Still not a whole lot of stopping until I just about hit the floor.

**nightmare** · 05-01-2009, 04:13 PM

yeah, if you've driven it several miles without fluid in the master, then you probably have air in the line after filling it back up. best to go ahead and bleed the brakes to get the correct pressure.

Longhorn Man · 05-01-2009, 04:18 PM

odds are you need a master cylinder. Look at the inside of the wheels. If they are dry, no oil lookin fluid all over the inside of them, then you might as well just go buy a new master cylinder, they are dirt cheap.
When you get it, read the paper in the box about adjusting the pushrod.
If you have any hissing and it is not from the check valve, then a new booster will be needed too. The 3/4 ton and one ton units can be hard to diagnose sometimes. Does the engine idle funny at all?

ChicoSmith · 05-01-2009, 04:26 PM

Just replaced the master cylinder about a month ago, hissing is gone with the new valve grommet part.

No fluid in the wheels from the wheel cylinders or around booster, and the master cylinder never got to the point of pulling air.

Yes it does idle a little rough but seems to be smoother with the new check valve. I could have been losing fluid through the old one?

No paper in the box about adjusting the pushrod.

Longhorn Man · 05-01-2009, 05:24 PM

For grins, fire up the truck, unplug that vac line, and put your thumb over the host to block off the suction. See if it idles better. You may have a bad booster.
Brake fluid gets in there and eats the rubber diaphram.

05-01-2009, 02:41 PM	#1
ChicoSmith When in doubt throttle out!!! Join Date: Feb 2009 Location: Chico, CA Posts: 22	68 c20 Brake Booster Hiss I have a leak near the rubber grommet where the plastic check valve inserts from the vacuum. I noticed fluid missing in the master cylinder reservoir closest to the cab and no pressure in the pedal during braking. Any words of wisdom from those who've dealt with this problem in the past.

05-01-2009, 03:00 PM	#2
sleepertruck72 Registered User Join Date: Nov 2007 Location: Turlock California 95380 Posts: 2,075	Re: 68 c20 Brake Booster Hiss My 72 C20 had the same problem with the reservoir closest to the firewall. I changed my rear wheel cylinders and I dont have that problem anymore. As for the check valve, If its not squared up against the booster it will hiss. Try replacing the vacuum line and leave it long enough to where the check valve sits flat against the booster.I would probly replace the check valve while youre at it. __________________ 72 C20 Longbed 05 1500 Z71

05-01-2009, 04:11 PM	#4
ChicoSmith When in doubt throttle out!!! Join Date: Feb 2009 Location: Chico, CA Posts: 22	Re: 68 c20 Brake Booster Hiss Thanks guys. Replaced the valve at lunch today and filled up the master cylinder, took it for a tester and it was a little better. Still not a whole lot of stopping until I just about hit the floor.

05-01-2009, 04:18 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: 68 c20 Brake Booster Hiss odds are you need a master cylinder. Look at the inside of the wheels. If they are dry, no oil lookin fluid all over the inside of them, then you might as well just go buy a new master cylinder, they are dirt cheap. When you get it, read the paper in the box about adjusting the pushrod. If you have any hissing and it is not from the check valve, then a new booster will be needed too. The 3/4 ton and one ton units can be hard to diagnose sometimes. Does the engine idle funny at all?

05-01-2009, 04:26 PM	#7
ChicoSmith When in doubt throttle out!!! Join Date: Feb 2009 Location: Chico, CA Posts: 22	Re: 68 c20 Brake Booster Hiss Just replaced the master cylinder about a month ago, hissing is gone with the new valve grommet part. No fluid in the wheels from the wheel cylinders or around booster, and the master cylinder never got to the point of pulling air. Yes it does idle a little rough but seems to be smoother with the new check valve. I could have been losing fluid through the old one? No paper in the box about adjusting the pushrod.

05-01-2009, 05:24 PM	#8
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: 68 c20 Brake Booster Hiss For grins, fire up the truck, unplug that vac line, and put your thumb over the host to block off the suction. See if it idles better. You may have a bad booster. Brake fluid gets in there and eats the rubber diaphram.



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