REAR DISK CONVERSION....

[Mmo]

hey fellas
ive read a few of the rear disk brake conversion threads, re to swap or not to swap the master cylinder. ive a new one that came with my truck. putting el dorado calipers out back for the mechanical E brake option, wilwoods already clamping on the front disks.

looking at my attached MC, any words of wisdom? we have to run the truck up on jack stands anyway to check for drive shaft wobbling, so will experiment with putting the brake on and see if they stop equal and quickly. then a test drive....

 

[6872xtc]

I'm not familiar with the company name cast into that master cylinder. So I can't help you. It seems to depend on what the pedal ratio is and the bore diameter of the master cylinder as to what works. Without knowing specific information, you may be in for some trial and error. When I converted my '70 K10 to rear disc, I ended up going to a Corvette master cylinder with hydro-boost. Part of my issue was trying to keep it tucked back out of the way of the A/C compressor, since I have it on the driver side. It stops fine now, but the pedal pressure feels somewhat high to me.

 

[Mmo]

I'm not familiar with the company name cast into that master cylinder. So I can't help you. It seems to depend on what the pedal ratio is and the bore diameter of the master cylinder as to what works. Without knowing specific information, you may be in for some trial and error. When I converted my '70 K10 to rear disc, I ended up going to a Corvette master cylinder with hydro-boost. Part of my issue was trying to keep it tucked back out of the way of the A/C compressor, since I have it on the driver side. It stops fine now, but the pedal pressure feels somewhat high to me.

thanks man. i went ahead and bought the Summit master cylinder proportioning valve. seems a good piece of kit and only $90. haven't received yet, and will likely try without just to get a baseline. my limited understanding is that the smaller wheel cylinder for drums needs more pressure than the larger calliper piston. i believe that drum master cylinders also supply a little constant pressure to the rear as well; i reckon that's a problem for my impeding rear disks... gasp?

 

[bp71k5]

Looks like you already have the updated proportioning valve? I don’t thing the master cylinder has the residual pressure valves in it, but the proportioning valve did. Might want to check me on that. The disc master cylinders are needed for discs cause of the increased volume of fluid they need for the self adjusting calipers.

 

[Mmo]

Looks like you already have the updated proportioning valve? I don’t thing the master cylinder has the residual pressure valves in it, but the proportioning valve did. Might want to check me on that. The disc master cylinders are needed for discs cause of the increased volume of fluid they need for the self adjusting calipers.

this MC and proportioning valve kit was always on the truck, but new when i bought. but you're saying it a better way than I; it's the proportioning valve that keeps residual pressure for rear drum brakes. so not desirable on my impending rear disks.
re your comment about disk MCs, this is due the brake pads wearing and the fluid level going down in the reservoir, the rear one being smaller on rear drums. um, ya?
curiouser and curiouser

 

[Blazooki]

Disc brakes require more fluid movement than drums to work. The factory proportioning valve can keep that from happening when you switch from drums to rear discs and is probably the biggest cause of issues when switching to rear dics. Most people remove the factory valve all together. They put in an after market adjustable valve inline and use that to adjust front/rear bias. like this one:

https://www.jegs.com/i/JEGS/555/630...QobChMI37_DoqSw4wIVEdRkCh18sANwEAQYASABEgKmjf

here's a good article on options
http://www.fourwheeler.com/how-to/transmission-drivetrain/129-1103-14-bolt-disc-brake-conversion/

 

[bp71k5]

re your comment about disk MCs, this is due the brake pads wearing and the fluid level going down in the reservoir, the rear one being smaller on rear drums. um, ya?
curiouser and curiouser

Yes.

 

[bp71k5]

Disc brakes require more fluid movement than drums to work. The factory proportioning valve can keep that from happening when you switch from drums to rear discs and is probably the biggest cause of issues when switching to rear dics. Most people remove the factory valve all together. They put in an after market adjustable valve inline and use that to adjust front/rear bias. like this one:

https://www.jegs.com/i/JEGS/555/630...QobChMI37_DoqSw4wIVEdRkCh18sANwEAQYASABEgKmjf

here's a good article on options
http://www.fourwheeler.com/how-to/transmission-drivetrain/129-1103-14-bolt-disc-brake-conversion/

If I was going to do it again, I’d also do it that way. The rear disc prop valve I’m not sure works all that well, or at least with bigger than normal tires.

 

[Mmo]

Thanks fellas. Reckon the entire stock proportioning valve system comes out and i'll install the new one.
i'll leave my new stock master cylinder in there and she how she stops.
be starting next weekend....

 

[GMCPaul]

The master cylinder on your Blazer doesn't need replaced when adding rear disc's. The only thing that needs replaced is the proportioning valve. Currently you have a disc / drum and you need disc / disc. The prop valve is what has the residual pressure valve in it that kept a small amount of pressure on rear drum brakes. The master cylinder currently on yours is from Performance Online and is there 1" bore corvette style.
Paul Jr @ GMCPauls

 

[Mmo]

getting there, for my rear disk brake conversion. ive the wilwood proportioning valve but can't connect it to the 1/4" rear brake line. so haven't installed.
the connector is too big, and i can't find an adaptor. see the proportioning valve that came with my truck in the top photo.

here's my question: see that there brass fitting in my hand, which connects between the rear 1/4" brake line and the proportioning valve (aftermarket) that came with the truck, when i bought it. you can see through it, tiny little hole. is this the 10psi check valve that keeps the pressure to the rear drums?

i can't even fit the rear 1/4" connector directly into the proportioning valve that came with my truck. may be an adaptor that can bypass this brass (10psi valve?)

sigh....

 

[bp71k5]

Shouldn’t need the thing in your hand. Not sure what to say on the lines, mine was fairly plug-n-play. Maybe the master cylinder is non-standard?

 

[Mmo]

rear disk brake blues...

so we got the proportioning valve in, ran new 3/16 lines and bled the calipers several times. but still not enough pedal pressure or stopping power. the front brakes get much hotter than the rears, which isn't surprising.

so need new master cylinder as suspected. ive looked at wilwoods and cheap stock ones. but the wilwood may need an adjustable rod. im happy buying the whole booster unit and MC as a kit. everything else is Wilwood, happy sticking wth this brand.

anybody recommend a now hassle MC part number, with booster?

just want to bleed and bolt.

cheers

 

[Mmo]

my pal at motor city K5 offers this here hydro boost option. taps into the hydraulic steer pump. he highly recommends, and knows what he's talking about.
anyone have experience with the install?
should i upgrade my hydraulic steer pump to what?!

 

[6872xtc]

I ended up going hydro-boost on my '70 K10 after doing the rear disc swap. Part of my reason was because I need to tuck it back a little compared to the factory booster. I needed the space for my A/C compressor lines.
Mine has worked fine with the Corvette master cylinder, but it has pedal feel that is firmer than I would like from an assisted system. Trying to keep it 100% clean and free of oil residue seems impossible, but I may be missing something small.

 

[Mmo]

I ended up going hydro-boost on my '70 K10 after doing the rear disc swap. Part of my reason was because I need to tuck it back a little compared to the factory booster. I needed the space for my A/C compressor lines.
Mine has worked fine with the Corvette master cylinder, but it has pedal feel that is firmer than I would like from an assisted system. Trying to keep it 100% clean and free of oil residue seems impossible, but I may be missing something small.

 

[Mmo]

strange that hydro boost would have firm pedal feel. apparently vacuum boost is 500 psi, hydro boost is 2000 psi.
does it get oil residue from the hydraulic pump? elaborate on the 100% clean...?

 

[6872xtc]

strange that hydro boost would have firm pedal feel. apparently vacuum boost is 500 psi, hydro boost is 2000 psi.
does it get oil residue from the hydraulic pump? elaborate on the 100% clean...?

I am guessing that the firm feel is because of the master cylinder bore size. I never tried changing it for another as I have good brakes, just firm which is better than having too much pedal travel.

Mine is always damp looking after heat cycles and a few miles. I am not sure if it is a fitting or the seals on the booster. I never found it for sure.
I was told to be careful about overtorquing the fittings in the booster, so it may be a seep there.
I got mine from Vanco.

 

[Mmo]

so it leaks a little hydraulic fluid, not the brake fluid. 2000psi tough to contain.

 

[6872xtc]

so it leaks a little hydraulic fluid, not the brake fluid. 2000psi tough to contain.

Correct, hydraulic oil. But 2000 psi isn't that much pressure to contain. I may have just scuffed an O-ring on a connection. Mine has adapters to convert to AN fittings.