Vacuum loss: Brake Booster or Master Cylinder - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Austin C10 · 01-04-2006, 03:02 PM

I've discovered the source of my brake warning dash light: There is a vacuum leak that is evident (hissing) between the brake booster and the master cylinder. I'm not sure of the culprit yet, because I'm not sure about the best way to correct the problem.

I can say that I can stop the leak by pinching a hole that goes from one side of the connection between the booster and MC. I'm not sure about the purpose of this hole, but it is clearly intended to be there.

Where is vacuum usually lost? The brake booster or the MC? If its the booster, is it easier to replace the booster or rebuild. Also, could the leak be coming from that connection? How do I fix that?

Tx Firefighter · 01-04-2006, 03:14 PM

There is no vacuum to the master cylinder, only to the booster itself.

You diaghram in your booster has apparantly ruptured. Don't attempt to dissasemble it. You can buy a reman unit at a local parts store for well under 100 dollars.

sneakysnake · 01-04-2006, 03:40 PM

A leaking brake booster is caused by a tear in the diaphram in the booster(can be rebuilt but probably cheaper to just replace).The mastercylinder has fluid in it only. The hole in question is for if the plunger seal goes out the fluid will weep out and not ruin the booster.
So if your light is on you have a brake problem with a bad booster as an extra,maybe caused by a leaking mastercylinder.Pull the M/C and check for leaking fluid,if dry then you need to check your (pads and shoes) for wear and possibly bleed the lines.

Austin C10 · 01-04-2006, 04:34 PM

It sounds like both of you are saying that the vacuum loss is from the booster, which makes sense. Are you also saying that the pressure differential switch in the metering/proportioning valve should be unaffected by the loss of vacuum? It sounds like the the booster repair isn't all I need.

The cost difference between the booster alone and the booster with a reman'd MC is not that great. Other than the $20, is there any reason to not just replace both when the booster goes bad?

matthufham · 01-04-2006, 05:13 PM

both would be great, alot less work involved.

01-04-2006, 03:02 PM	#1
Austin C10 1972 C10 Cheyenne LWB Join Date: Dec 2005 Location: Redwood City, CA Posts: 41	Vacuum loss: Brake Booster or Master Cylinder I've discovered the source of my brake warning dash light: There is a vacuum leak that is evident (hissing) between the brake booster and the master cylinder. I'm not sure of the culprit yet, because I'm not sure about the best way to correct the problem. I can say that I can stop the leak by pinching a hole that goes from one side of the connection between the booster and MC. I'm not sure about the purpose of this hole, but it is clearly intended to be there. Where is vacuum usually lost? The brake booster or the MC? If its the booster, is it easier to replace the booster or rebuild. Also, could the leak be coming from that connection? How do I fix that? __________________ 1972 LWB Cheyenne C10 350 TurboHydramatic

01-04-2006, 03:14 PM	#2
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	Re: Vacuum loss: Brake Booster or Master Cylinder There is no vacuum to the master cylinder, only to the booster itself. You diaghram in your booster has apparantly ruptured. Don't attempt to dissasemble it. You can buy a reman unit at a local parts store for well under 100 dollars. __________________ I'm on the Instagram- @Gearhead_Kevin

01-04-2006, 03:40 PM	#3
sneakysnake It's a catastrophic success. Join Date: Jan 2003 Location: Oklahoma Posts: 2,077	Re: Vacuum loss: Brake Booster or Master Cylinder A leaking brake booster is caused by a tear in the diaphram in the booster(can be rebuilt but probably cheaper to just replace).The mastercylinder has fluid in it only. The hole in question is for if the plunger seal goes out the fluid will weep out and not ruin the booster. So if your light is on you have a brake problem with a bad booster as an extra,maybe caused by a leaking mastercylinder.Pull the M/C and check for leaking fluid,if dry then you need to check your (pads and shoes) for wear and possibly bleed the lines.

01-04-2006, 04:34 PM	#4
Austin C10 1972 C10 Cheyenne LWB Join Date: Dec 2005 Location: Redwood City, CA Posts: 41	Re: Vacuum loss: Brake Booster or Master Cylinder It sounds like both of you are saying that the vacuum loss is from the booster, which makes sense. Are you also saying that the pressure differential switch in the metering/proportioning valve should be unaffected by the loss of vacuum? It sounds like the the booster repair isn't all I need. The cost difference between the booster alone and the booster with a reman'd MC is not that great. Other than the $20, is there any reason to not just replace both when the booster goes bad? __________________ 1972 LWB Cheyenne C10 350 TurboHydramatic

01-04-2006, 05:13 PM	#5
matthufham Registered User Join Date: Feb 2005 Location: Carl Junction, Missouri Posts: 2,061	Re: Vacuum loss: Brake Booster or Master Cylinder both would be great, alot less work involved.

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