4 wheel disc conversion question (M/C) - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kbeefy · 08-29-2005, 01:26 AM

Okay, so I've got this 70 'burban k20 and the brakes were crap. For several different reasons I wound up putting later model axles under it (D44 and 14bFF) along with a rear disc brake conversion. So now, I have basically mid seventies k20 front brake calipers all around, but still have the stock master cylinder (it's power brakes, btw). They work pretty well, but the pedal is pretty soft and I can push it all the way to the floor. I'm guessing that this little master cylinder isn't pushing enough fluid to work the calipers really well, but it is working fairly evenly, due to the front and rear portions of the master cylinder being about equal.

So, after that enthralling introduction, my questionis this: Does anyone know of a master cylinder I can install that has a larger bore to handle the disc brakes (higher flow, more pressure etc) and is equal front to rear (not like one w/ front disc - rear drum)?

Thanks in advance for any advice!

Mudder · 08-29-2005, 02:52 AM

I have nearly the same set up as you. I am running a 67 drum/drum master. When I first did the brakes it was a good firm petal. I have changed to an Auto Zone master and they are soft like you said yours were. I think mine need the fluid removed and replaced with new and a good bleeding. LMC does have a M/C for disk set ups now I see.

*FormerMember* · 08-29-2005, 09:50 AM

Actually you need to swap out the proportioning valve.

the drum rear valve has a check valve in it that keeps the hydraulic fluid from coming back when you let off the brakes. That keeps the shoes in place so the next time you stop, the shoes don't have to travel out to meet the drum.

In a disk application, you want the fluid to travel as the caliper floats back and forth. You also need the bleed the heck out of the system. Most people put the caliper in the worst possible location for bleeding the brakes. I suggest bleeding the caliper off the rotor, so the air flows out the bleeder valve instead of sitting in the "top" of the caliper.

the GM part number for the disk/disk proportioning valve is # 1257225 P30-60 truck that was a one ton ether lunch or utility truck and had 4 wheel disk brakes.

Can also buy them from inline tube or on Ebay.

nu2-72 · 08-29-2005, 11:15 AM

The 76 Corvette master cylinder is set up for 4 wheel disc, has a dual setup with different sized reservoirs and has a larger bore size for more fluid flow. Check into one of those. You will need the diferent size flow to match the prop valve.

kbeefy · 08-29-2005, 11:34 AM

thanks for the good info everyone, I'll check into the prop valve and a vette M/C

*FormerMember* · 08-29-2005, 11:36 AM

When I looked into the Corvette MC, I couldn't get it to line up with the stock 3/4 ton booster I was using.

Is there a different one?

nu2-72 · 08-29-2005, 01:21 PM

I do not know of a different one. Several people on here have done this although I cannot remember anyone by name right now. If you are in no hurry, give it some time and people will respond. There is some great info on here. I will do some checking as well. I have all the info at home somewhere.

nu2-72 · 08-29-2005, 01:22 PM

Just noticed, kbeefy, you auto-x. We did for a few years with SCCA and the Corvette clubs. Where are you located?

kbeefy · 08-29-2005, 04:59 PM

Quote:

Originally Posted by nu2-72

Just noticed, kbeefy, you auto-x. We did for a few years with SCCA and the Corvette clubs. Where are you located?

I'm in alaska (anchorage, to be precise). Haven't actually raced anything yet, just had this car and decided to build a racer out of it. Time's been in short supply and progress has been nill for the last year. Theres a pretty cool race held up here in march, I might try to get enough done to be competitive there.
Theres a SCCA chapter and Porsche club up here that do most of the auto-x stuff.

Musclerodz · 08-29-2005, 05:38 PM

Put a hydroboost unit on it off a 3/4 or 1 ton. Should give you brakes galore.

Mike

Mudder · 08-29-2005, 02:39 PM

I have heard too that there is a vette mc that will work but not sure of the year.

*FormerMember* · 08-29-2005, 02:46 PM

Quote:

Originally Posted by Mudder67

I have heard too that there is a vette mc that will work but not sure of the year.

I tried a 67, a 74, a 79 and an 85. Couldn't get the right throw on any of them. Even tried to use their boosters and did not want to modify my pedals.

BOY was my parts person pissed!

08-29-2005, 01:26 AM	#1
kbeefy Registered User Join Date: Jan 2005 Location: Alaska Posts: 39	4 wheel disc conversion question (M/C) Okay, so I've got this 70 'burban k20 and the brakes were crap. For several different reasons I wound up putting later model axles under it (D44 and 14bFF) along with a rear disc brake conversion. So now, I have basically mid seventies k20 front brake calipers all around, but still have the stock master cylinder (it's power brakes, btw). They work pretty well, but the pedal is pretty soft and I can push it all the way to the floor. I'm guessing that this little master cylinder isn't pushing enough fluid to work the calipers really well, but it is working fairly evenly, due to the front and rear portions of the master cylinder being about equal. So, after that enthralling introduction, my questionis this: Does anyone know of a master cylinder I can install that has a larger bore to handle the disc brakes (higher flow, more pressure etc) and is equal front to rear (not like one w/ front disc - rear drum)? Thanks in advance for any advice! __________________ 70 k20 'burban (latest aqusition) 91 k1500 xcab lb 99 f350 (sold) 83 cj7 (huge and for sale) 82 rx7 (auto-x project) 74 VW superbettle 04 summit 800 151 & 05 renegade 600 guns, flying, camping, biking, 4-byin, bbq's and beer!

08-29-2005, 11:15 AM	#4
nu2-72 Senior Member Join Date: Apr 2001 Location: Amazonia, Mo. USA Posts: 2,821	The 76 Corvette master cylinder is set up for 4 wheel disc, has a dual setup with different sized reservoirs and has a larger bore size for more fluid flow. Check into one of those. You will need the diferent size flow to match the prop valve. __________________ 72 3/4 4x4 350 4spd 72 3/4 4x4 350 auto 2004 Duramax Silverado 4x4 2006 Duramax Crew cab 4x4 2010 Harley Ultra Limited 2011 Harley Street Glide

08-29-2005, 11:34 AM	#5
kbeefy Registered User Join Date: Jan 2005 Location: Alaska Posts: 39	thanks for the good info everyone, I'll check into the prop valve and a vette M/C __________________ 70 k20 'burban (latest aqusition) 91 k1500 xcab lb 99 f350 (sold) 83 cj7 (huge and for sale) 82 rx7 (auto-x project) 74 VW superbettle 04 summit 800 151 & 05 renegade 600 guns, flying, camping, biking, 4-byin, bbq's and beer!

08-29-2005, 01:21 PM	#7
nu2-72 Senior Member Join Date: Apr 2001 Location: Amazonia, Mo. USA Posts: 2,821	I do not know of a different one. Several people on here have done this although I cannot remember anyone by name right now. If you are in no hurry, give it some time and people will respond. There is some great info on here. I will do some checking as well. I have all the info at home somewhere. __________________ 72 3/4 4x4 350 4spd 72 3/4 4x4 350 auto 2004 Duramax Silverado 4x4 2006 Duramax Crew cab 4x4 2010 Harley Ultra Limited 2011 Harley Street Glide

08-29-2005, 01:22 PM	#8
nu2-72 Senior Member Join Date: Apr 2001 Location: Amazonia, Mo. USA Posts: 2,821	Just noticed, kbeefy, you auto-x. We did for a few years with SCCA and the Corvette clubs. Where are you located? __________________ 72 3/4 4x4 350 4spd 72 3/4 4x4 350 auto 2004 Duramax Silverado 4x4 2006 Duramax Crew cab 4x4 2010 Harley Ultra Limited 2011 Harley Street Glide

08-29-2005, 02:52 AM	#2
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	I have nearly the same set up as you. I am running a 67 drum/drum master. When I first did the brakes it was a good firm petal. I have changed to an Auto Zone master and they are soft like you said yours were. I think mine need the fluid removed and replaced with new and a good bleeding. LMC does have a M/C for disk set ups now I see.

08-29-2005, 09:50 AM	#3
FormerMember Account Suspended Join Date: Feb 2004 Posts: 3,051	Actually you need to swap out the proportioning valve. the drum rear valve has a check valve in it that keeps the hydraulic fluid from coming back when you let off the brakes. That keeps the shoes in place so the next time you stop, the shoes don't have to travel out to meet the drum. In a disk application, you want the fluid to travel as the caliper floats back and forth. You also need the bleed the heck out of the system. Most people put the caliper in the worst possible location for bleeding the brakes. I suggest bleeding the caliper off the rotor, so the air flows out the bleeder valve instead of sitting in the "top" of the caliper. the GM part number for the disk/disk proportioning valve is # 1257225 P30-60 truck that was a one ton ether lunch or utility truck and had 4 wheel disk brakes. Can also buy them from inline tube or on Ebay.

08-29-2005, 11:36 AM	#6
FormerMember Account Suspended Join Date: Feb 2004 Posts: 3,051	When I looked into the Corvette MC, I couldn't get it to line up with the stock 3/4 ton booster I was using. Is there a different one?

08-29-2005, 05:38 PM	#10
Musclerodz Senior Member Join Date: Aug 2005 Location: OKC, OK Posts: 2,365	Put a hydroboost unit on it off a 3/4 or 1 ton. Should give you brakes galore. Mike __________________ Mike Redpath

08-29-2005, 02:39 PM	#11
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	I have heard too that there is a vette mc that will work but not sure of the year.



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