what happens if you don't change it... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**'70_402** · 03-23-2005, 11:17 AM

What will happen if you swap your brakes over to front disc and don't change out you prop valve for one that was made for disc drum... will it damage anything.. just curious...

Thanks

Rick

JimKshortstep4x4 · 03-23-2005, 11:22 AM

I don't see that the system will be damaged. The prop valve is there to keep the rear brakes from locking up before the fronts do. Normal stops may not be affected but I would suggest that the proper prop valve be installed with the upgrade.

Jim

Bobalouie · 03-23-2005, 11:53 AM

Nothing will be damaged, but you dont get the very best stopping performance from the swap. I have a buddy that did the swap from drums to disk on his 70, and he didnt change the prop valve or master cylinder. When he slams on the brakes it will put you thru the windshield.

Gary Lee · 03-23-2005, 08:39 PM

Quote:

Originally Posted by 70'_402

What will happen if you swap your brakes over to front disc and don't change out you prop valve for one that was made for disc drum... will it damage anything.. just curious...

Thanks

Rick

Rick, I don't know... but knowinghuman nature, if something happens like an accident due to brake malfunction... after your significant other is done, with you... twenty-five peeple are going to say you should've changed it... a couple might even laugh at you if no one got hurt... honestly I haven't the foggiest...

Good Luck
Gary

**JIMs70GMC** · 03-23-2005, 09:44 PM

I put a 70 c20 cab on a 71 K20 chassis and did not change the MC or Prop valve. It has work great, been about 6000 miles/10 months so far.

Blue4x4 · 03-23-2005, 09:50 PM

I did the drum to disc swap and ran the old prop. valve for a year, and when I change to the new valve .....not much of a difference.

3 on the tree · 03-23-2005, 10:36 PM

I think the only time it could be a problem is under slippery conditions. Disc brakes require alot more fluid transfer than drums, so at least change to the proper master cylinder.

Longhorn Man · 03-23-2005, 10:38 PM

Disc brakes work on a much higher line pressure than drum brakes. By using the old distibution valve instead of the proper proportioning valve, you are not getting the full effect of the swap. The front discs are only getting a fraction of the pressure needed to operate them. This, in turn wears your rear shoes quicker, and can cause premature rear wheel lock up....esp in turns or on slick pavement.

03-23-2005, 11:17 AM	#1
'70_402 Senior Member Join Date: Dec 2003 Location: Pampa, Texas Posts: 1,554	what happens if you don't change it... What will happen if you swap your brakes over to front disc and don't change out you prop valve for one that was made for disc drum... will it damage anything.. just curious... Thanks Rick __________________ https://www.ebay.com/itm/295742064443

03-23-2005, 11:53 AM	#3
Bobalouie Fetzer Valve Technician Join Date: Jun 2003 Location: Stillwater, OK Posts: 205	Nothing will be damaged, but you dont get the very best stopping performance from the swap. I have a buddy that did the swap from drums to disk on his 70, and he didnt change the prop valve or master cylinder. When he slams on the brakes it will put you thru the windshield. __________________ 1972 Chevy K10 SWB

03-23-2005, 09:44 PM	#5
JIMs70GMC user # 2756 Join Date: Jun 2001 Location: Chesapeake, Virginia Posts: 4,612	I put a 70 c20 cab on a 71 K20 chassis and did not change the MC or Prop valve. It has work great, been about 6000 miles/10 months so far. __________________ 1970 K25, 8' stepside bed 350/465/205 44 up front, 60 in the rear 4.10s rolling on 33" Dunlop MTs 1986 K5, 350/465/208 Dana 60/14 bolt from a cucv 36" Super Swampers TSL/SX 1983 K20 w/ CUCV axles, 350/700R4/208 sitting on 37" Goodyears 1986 M1031 6.2 diesel, TH400/NP205 locker in the rear and a LS in the front, all stock for now..... 1986 K30, 350/400/205 dana 60 and 14 bolt. I kept the drivetrain. Body/bad and chassis are gone. 1981 K30, 350/465/205 dana 60 and dually 14 bolt. Has a G80, and a flat bed. Going to replace the flat bed. 1985 K20, 350/400/208 10 bolt and SF 14 bolt. I wonder where I can find some 1 tons. Hmmmmm

03-23-2005, 11:22 AM	#2
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	I don't see that the system will be damaged. The prop valve is there to keep the rear brakes from locking up before the fronts do. Normal stops may not be affected but I would suggest that the proper prop valve be installed with the upgrade. Jim

03-23-2005, 09:50 PM	#6
Blue4x4 Senior Member Join Date: Mar 2003 Location: Sparks, NV Posts: 790	I did the drum to disc swap and ran the old prop. valve for a year, and when I change to the new valve .....not much of a difference.

03-23-2005, 10:36 PM	#7
3 on the tree Registered User Join Date: Oct 2004 Posts: 393	I think the only time it could be a problem is under slippery conditions. Disc brakes require alot more fluid transfer than drums, so at least change to the proper master cylinder.

03-23-2005, 10:38 PM	#8
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Disc brakes work on a much higher line pressure than drum brakes. By using the old distibution valve instead of the proper proportioning valve, you are not getting the full effect of the swap. The front discs are only getting a fraction of the pressure needed to operate them. This, in turn wears your rear shoes quicker, and can cause premature rear wheel lock up....esp in turns or on slick pavement.



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