Drum to Disk brake help needed - The 1947 - Present Chevrolet & GMC Truck Message Board Network

steves1967 · 08-23-2010, 11:55 PM

I'm helping my buddy with his 70 GMC swb, he got a disk brake conversion kit from the Truck Shop, and a power booster from Kragen Auto Parts. He put the whole thing together and asked me to help bleed the brakes for him. Seems that the truck will stop, but he needs to pump the pedal a couple times to get a firm pedal. I bench bled the master cylinder, and I bleed each wheel 3 times around, no air bubbles in the system, yet he still has a soft pedal until he pumps it a couple times. Am I missing something here? Engien makes 17" of vacuum, the m/c isnt leaking. Attached is a pic of his setup, am I missing something?
Thanks!!

cheyenne shortstep · 08-24-2010, 12:49 AM

May need to adjust the pushrod.

ERASER5 · 08-24-2010, 01:54 AM

Push rod adjustment is critical to good brakes. The booster pin needs to be set correctly too. I had to do both for mine. To get the pin adjusted just right, I had to remove it and grind a little.

Do you have a residual valve inline? That looks like a disc/disc MC. That's fine if it is, but a 10 lb. residual valve is needed for the rear drums or soft pedal will occur. The difference is night and day.

Here is a linc to Master Power Brake's Tech support. This page helped me.
http://www.mpbrakes.com/technical-su...sc-drum-01.cfm

steves1967 · 08-24-2010, 01:56 AM

Thanks for the info, he doesnt have the residual valve yet. I have the pushrod adjusted pretty well, about 1/2" travel before the pushrod contacts the m/c piston. Is that too loose?

steves1967 · 08-24-2010, 01:58 AM

Also I believe he has the front port on the m/c plumbed to his front brakes, is this correct?

ERASER5 · 08-24-2010, 05:27 AM

Quote:

Originally Posted by steves1967

Thanks for the info, he doesnt have the residual valve yet. I have the pushrod adjusted pretty well, about 1/2" travel before the pushrod contacts the m/c piston. Is that too loose?

Yes. That is too loose. The booster pin should be 1/64" from the MC.

Quote:

Originally Posted by steves1967

Also I believe he has the front port on the m/c plumbed to his front brakes, is this correct?

For that type of MC, the front should be the front brakes.

steves1967 · 08-24-2010, 12:37 PM

Great tanks, is there anywhere in particular that I should mount the pressure valve?

ERASER5 · 08-24-2010, 01:48 PM

I put mine along the frame rail.

68 TT · 08-24-2010, 03:21 PM

It is possible the push rod coming out of the booster is too short for the master cylinder and it is only getting partial travel of the master cylinder piston as a result.

ERASER5 · 08-24-2010, 09:01 PM

Which is why I said to check the booster pin, gave a linc saying the same and related that I had to adjust mine. The pin is screw adjustable.

08-23-2010, 11:55 PM	#1
steves1967 Registered User Join Date: Dec 2001 Location: Manteca, Ca USA Posts: 305	Drum to Disk brake help needed I'm helping my buddy with his 70 GMC swb, he got a disk brake conversion kit from the Truck Shop, and a power booster from Kragen Auto Parts. He put the whole thing together and asked me to help bleed the brakes for him. Seems that the truck will stop, but he needs to pump the pedal a couple times to get a firm pedal. I bench bled the master cylinder, and I bleed each wheel 3 times around, no air bubbles in the system, yet he still has a soft pedal until he pumps it a couple times. Am I missing something here? Engien makes 17" of vacuum, the m/c isnt leaking. Attached is a pic of his setup, am I missing something? Thanks!! __________________ '71 Chevy 3/4 Ton Longbed '22 Chevy Tahoe RST '67 Firebird- 8 sec street car '24 GMC CC 4x4 Duramax Dually

08-24-2010, 12:49 AM	#2
cheyenne shortstep Registered User Join Date: Jul 2010 Location: Vista Ca. Posts: 2,405	Re: Drum to Disk brake help needed May need to adjust the pushrod.

08-24-2010, 01:54 AM	#3
ERASER5 Registered User Join Date: Jun 2006 Location: Indiana Posts: 4,859	Re: Drum to Disk brake help needed Push rod adjustment is critical to good brakes. The booster pin needs to be set correctly too. I had to do both for mine. To get the pin adjusted just right, I had to remove it and grind a little. Do you have a residual valve inline? That looks like a disc/disc MC. That's fine if it is, but a 10 lb. residual valve is needed for the rear drums or soft pedal will occur. The difference is night and day. Here is a linc to Master Power Brake's Tech support. This page helped me. http://www.mpbrakes.com/technical-su...sc-drum-01.cfm __________________ '70 GMC C1500 LWB Power disc brakes. WooHoo! Posi 6 Lug Dana 60

08-24-2010, 01:56 AM	#4
steves1967 Registered User Join Date: Dec 2001 Location: Manteca, Ca USA Posts: 305	Re: Drum to Disk brake help needed Thanks for the info, he doesnt have the residual valve yet. I have the pushrod adjusted pretty well, about 1/2" travel before the pushrod contacts the m/c piston. Is that too loose? __________________ '71 Chevy 3/4 Ton Longbed '22 Chevy Tahoe RST '67 Firebird- 8 sec street car '24 GMC CC 4x4 Duramax Dually

08-24-2010, 01:58 AM	#5
steves1967 Registered User Join Date: Dec 2001 Location: Manteca, Ca USA Posts: 305	Re: Drum to Disk brake help needed Also I believe he has the front port on the m/c plumbed to his front brakes, is this correct? __________________ '71 Chevy 3/4 Ton Longbed '22 Chevy Tahoe RST '67 Firebird- 8 sec street car '24 GMC CC 4x4 Duramax Dually

08-24-2010, 12:37 PM	#7
steves1967 Registered User Join Date: Dec 2001 Location: Manteca, Ca USA Posts: 305	Re: Drum to Disk brake help needed Great tanks, is there anywhere in particular that I should mount the pressure valve? __________________ '71 Chevy 3/4 Ton Longbed '22 Chevy Tahoe RST '67 Firebird- 8 sec street car '24 GMC CC 4x4 Duramax Dually

08-24-2010, 01:48 PM	#8
ERASER5 Registered User Join Date: Jun 2006 Location: Indiana Posts: 4,859	Re: Drum to Disk brake help needed I put mine along the frame rail. __________________ '70 GMC C1500 LWB Power disc brakes. WooHoo! Posi 6 Lug Dana 60

08-24-2010, 03:21 PM	#9
68 TT Still plays with trucks Join Date: Jul 2009 Location: Spokane, WA Posts: 3,556	Re: Drum to Disk brake help needed It is possible the push rod coming out of the booster is too short for the master cylinder and it is only getting partial travel of the master cylinder piston as a result. __________________ miSSed opportunity - ground up creation of an AWD 1994 454 SS that never was http://67-72chevytrucks.com/vboard/s...d.php?t=795577 69 C10 shortbed stepside 400 small block - built like what a super sport truck could have been 69 K20 lwb TBI 350 4L60E NP208 14-bolt Dana-44 w/disc 68 Camaro SS / RS 500hp 439 inch roller cam big block 4L80E 79 Malibu TPI 350 4L60 w/ Z28 steering & sway bars

08-24-2010, 09:01 PM	#10
ERASER5 Registered User Join Date: Jun 2006 Location: Indiana Posts: 4,859	Re: Drum to Disk brake help needed Which is why I said to check the booster pin, gave a linc saying the same and related that I had to adjust mine. The pin is screw adjustable. __________________ '70 GMC C1500 LWB Power disc brakes. WooHoo! Posi 6 Lug Dana 60



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