Help! Master cylinder question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**ScottH** · 04-25-2003, 12:45 PM

I recently bought a 68 which had been previously converted to 5 lug all around with disc up front. I haven't driven the truck (usually helps to have a motor first), but previous owner (not the one who converted it) said the brakes worked fine.

It appears to have the original booster and master cylinder in it working with the front disc brakes. I'm a little concerned that this master cylinder, and possibly the proportioning valve, aren't the right ones for disc brakes. I reason I believe it may be the original master cylinder is because it is the small style (same size reservoirs).

If anyone can shed some light on this any info is appreciated. Also how can I tell if the proportioning valve is the right one?

Thanks!

**CPNE** · 04-25-2003, 01:39 PM

The prop valve will have a smaller diameter line leading to the front brakes if its a disk prop valve.

Southpa · 04-25-2003, 03:55 PM

Yes, the original master cylinder (for drums)will have both fluid reservoirs equal size. The disc style M/C has forward reservoir larger. Booster shouldn't be a problem but when replacing the master cyl. don't go with the single hold-down clip variety, especially the remans. I went thru 2 that leaked out the cap. Get the beefier one with the 2 hold down clips for the cap.

And also, you will need the prop. valve for a disc setup. They are hard to find and I'm not sure if later model trucks (73 on) use the same one.

**ScottH** · 04-26-2003, 10:24 PM

Thanks for all the info.

So will the drum master cylinder not work with the disc brakes? It is hooked and working as far as I know. Will it not work correctly?

Thanks again.

Southpa · 04-27-2003, 12:25 AM

More stopping power is required in the front than the rear when running discs (more pressure needed to push calipers than wheel cylinders, I guess). You can use a drum M/C on front discs but won't be as effective as the disc M/C. The larger front reservoir, working with the right proportioning valve, ensures you get the right proportion of braking power between front and rear.

**ScottH** · 04-27-2003, 11:37 AM

Thanks for all the help!!!

lukecp · 04-27-2003, 03:03 PM

I have a equal size MC on my truck, a '72 C10 with factory power disk brakes. When i bought the truck, the MC that was on it had a larger rear resivour, but the one i replaced it with has equal sized resivours. I don't think the brakes work as good as they should, could that be a reason? The guy at the parts store assured me that even though the MC's looked different, the new one would work just fine. This guy has lots of experience and usually knows his stuff....but could he be giving me bad advice?

Oh yeah, the original MC had one bail hoop holding down the cap, and it was pointing from the front of the truck to the brake booster. The one on it now has two bail hoops holding down the cap, and both point from the fenders to the motor.

**ScottH** · 04-27-2003, 10:27 PM

Ok, I finally got the chance to check and I do have a larger line going from the proportioning valve to the rear brakes. So should I change the master cylinder to have the larger reservoir in front?

Thanks.

Ed ke6bnl · 04-28-2003, 07:00 AM

Quote:

Originally posted by yellow72custom
I have a equal size MC on my truck, a '72 C10 with factory power disk brakes. When i bought the truck, the MC that was on it had a larger rear resivour, but the one i replaced it with has equal sized resivours. I don't think the brakes work as good as they should, could that be a reason? The guy at the parts store assured me that even though the MC's looked different, the new one would work just fine. This guy has lots of experience and usually knows his stuff....but could he be giving me bad advice?

Oh yeah, the original MC had one bail hoop holding down the cap, and it was pointing from the front of the truck to the brake booster. The one on it now has two bail hoops holding down the cap, and both point from the fenders to the motor.

dito dito, when reading your response I thought I wrote it. That is the exact experience that I went through with my bos truck. Ed ke6bnl

04-25-2003, 12:45 PM	#1
ScottH Senior Member Join Date: Sep 2002 Location: Hampstead, NC Posts: 8,190	Help! Master cylinder question I recently bought a 68 which had been previously converted to 5 lug all around with disc up front. I haven't driven the truck (usually helps to have a motor first), but previous owner (not the one who converted it) said the brakes worked fine. It appears to have the original booster and master cylinder in it working with the front disc brakes. I'm a little concerned that this master cylinder, and possibly the proportioning valve, aren't the right ones for disc brakes. I reason I believe it may be the original master cylinder is because it is the small style (same size reservoirs). If anyone can shed some light on this any info is appreciated. Also how can I tell if the proportioning valve is the right one? Thanks!

04-25-2003, 01:39 PM	#2
CPNE Resident Curmudgeon Join Date: Dec 1999 Location: NH Posts: 6,662	The prop valve will have a smaller diameter line leading to the front brakes if its a disk prop valve. __________________ Currently on or near the homestead: 67 Chevy SWB 2WD stepside 350/3 on tree (Pat's) 67 GMC SWB 2WD Fleet 402/auto (Brian's under construction) 67 Chevy 3/4 ton 2WD 402/auto (Business Hauler) 67 Chevy 1 ton dually 2WD 396/4 speed (Former business hauler, Needs TLC) 68 Chevy 1/2 ton Suburban 2WD 250 six/3 on tree (Brian's Needs TLC) 70 Chevy 3/4 ton 4WD 350/4 speed (Pat's - Disguised as a 68 GMC) 71 Chevy SWB stepside (Crushed by tree - parts donor) 72 Chevy 3/4 ton 4WD (Parts donor) 72 Chevy 3/4 ton 4WD Suburban (Parts Donor) 72 GMC 3/4 ton 4WD 292 six/4 speed (Mine - Disguised as a 67 GMC) 81 GMC 4WD Dually Dump Body 350/4 speed (Business Hauler) 82 Camaro Z/28 355/Super T-10 (Pat's toy) 93 Caprice 9C1 (Brian's Cop Car) 02 Toyota Camry (Reliable but a souless steel and plastic hulk) 2011 2SS RS Camaro M6 Factory Hurst Shifter Maybe I need to sell some of this crap Yet another Bozo with a sawz-all

04-25-2003, 03:55 PM	#3
Southpa Registered User Join Date: May 2000 Location: Victoria, BC, Canada Posts: 2,409	Yes, the original master cylinder (for drums)will have both fluid reservoirs equal size. The disc style M/C has forward reservoir larger. Booster shouldn't be a problem but when replacing the master cyl. don't go with the single hold-down clip variety, especially the remans. I went thru 2 that leaked out the cap. Get the beefier one with the 2 hold down clips for the cap. And also, you will need the prop. valve for a disc setup. They are hard to find and I'm not sure if later model trucks (73 on) use the same one. __________________ 1970 GMC 1500 Custom Original 350/TH350 Victoria, BC, Canada You can wish in one hand and crap in the other. See which one gets filled first.

04-27-2003, 12:25 AM	#5
Southpa Registered User Join Date: May 2000 Location: Victoria, BC, Canada Posts: 2,409	More stopping power is required in the front than the rear when running discs (more pressure needed to push calipers than wheel cylinders, I guess). You can use a drum M/C on front discs but won't be as effective as the disc M/C. The larger front reservoir, working with the right proportioning valve, ensures you get the right proportion of braking power between front and rear. __________________ 1970 GMC 1500 Custom Original 350/TH350 Victoria, BC, Canada You can wish in one hand and crap in the other. See which one gets filled first.

04-27-2003, 03:03 PM	#7
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	I have a equal size MC on my truck, a '72 C10 with factory power disk brakes. When i bought the truck, the MC that was on it had a larger rear resivour, but the one i replaced it with has equal sized resivours. I don't think the brakes work as good as they should, could that be a reason? The guy at the parts store assured me that even though the MC's looked different, the new one would work just fine. This guy has lots of experience and usually knows his stuff....but could he be giving me bad advice? Oh yeah, the original MC had one bail hoop holding down the cap, and it was pointing from the front of the truck to the brake booster. The one on it now has two bail hoops holding down the cap, and both point from the fenders to the motor. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

04-26-2003, 10:24 PM	#4
ScottH Senior Member Join Date: Sep 2002 Location: Hampstead, NC Posts: 8,190	Thanks for all the info. So will the drum master cylinder not work with the disc brakes? It is hooked and working as far as I know. Will it not work correctly? Thanks again.

04-27-2003, 11:37 AM	#6
ScottH Senior Member Join Date: Sep 2002 Location: Hampstead, NC Posts: 8,190	Thanks for all the help!!!

04-27-2003, 10:27 PM	#8
ScottH Senior Member Join Date: Sep 2002 Location: Hampstead, NC Posts: 8,190	Update Ok, I finally got the chance to check and I do have a larger line going from the proportioning valve to the rear brakes. So should I change the master cylinder to have the larger reservoir in front? Thanks.

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