New master and now no power brakes

[shawno72]

Ok I did a disc brake upgrade to my 68 swb. I already had a power booster and now it feels like it isn't working. I used a 1975 Corvette style master cylinder and the truck stops ok but the pedal is hard. Any ideas?

[SCHRUMGMC]

How do you Upgrade to newer style Brakes ?

[shawno72]

Originally Posted by SCHRUMGMC

How do you Upgrade to newer style Brakes ?

Went from drum to disc...

[SCHRUMGMC]

Oh simple as that then

[ulakovic22]

Just the front disks, upgraded? I'm guessing that you got a CPP master and prop valve combo for disk/drum or disk/disk, which ever you have? Bore size on the master, did you change the pushrod, bleed the brake system correctly?
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[shawno72]

I used a rebuilt 75 Corvette master with a 1" bore. Stock rebuilt booster that is workiń properly and I believe I bleed the system good. Nice firm pedal. Front disc upgrade only with new CPP disc/drum prop valve
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[buckskins4ever]

Also since you had the booster already, did you look to make sure it didnt have fluid in it? I had to replace mine due to this. And another is poor vacuum pressure. Where are you porting the vacuum for the booster from?

[shawno72]

Originally Posted by buckskins4ever

Also since you had the booster already, did you look to make sure it didnt have fluid in it? I had to replace mine due to this. And another is poor vacuum pressure. Where are you porting the vacuum for the booster from?

The booster is less than 6 months old and was working great with the drum brakes. Easy toe stops and no worries. There was no fluid in it and I checked it and it was holding vacuum when I removed the check valve.
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[shawno72]

My concern was maybe I should of used the 1 1/8 inch bore master but I figured the smaller bore would cause higher pressure.
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[shawno72]

After reading this I'm thinking I may need to switch to the larger 1 1/8 bore

http://67-72chevytrucks.com/vboard/s...d.php?t=561809

[ulakovic22]

Originally Posted by shawno72

After reading this I'm thinking I may need to switch to the larger 1 1/8 bore

http://67-72chevytrucks.com/vboard/s...d.php?t=561809

You have a 1" bore MC, if you go to a larger bore the pedal effort will increase because you are trying to move more fluid. If you had power drums before and just switched to power disks then your pushrod should be ok. Just make sure your vacuum source is good, hooked up, no leaks and that your brakes are bled well and you should be good.

[shawno72]

Originally Posted by tinsoldier

Did you properly bench bleed the master ? Not sure if that is a brand new (looks brand new from the pic). All the brand new master cylinder I get has a note saying to you have to bench bleed it or at least comes with 2 lines where you can bleed it in the car before installation of the hardlines.

Yes I did bench bleed.

Originally Posted by ulakovic22

You have a 1" bore MC, if you go to a larger bore the pedal effort will increase because you are trying to move more fluid. If you had power drums before and just switched to power disks then your pushrod should be ok. Just make sure your vacuum source is good, hooked up, no leaks and that your brakes are bled well and you should be good.

Thats kinda what I was thinking. I know enough about hydraulics to now smaller piston equals higher pressure. I am just at a loss and will try to bleed some more. Plus on my 72, it had manual brakes and all I did was add a booster and viola power brakes. Those come with 1 1/8 bore masters I believe.

Driving it to work this morning made me realize that I will get used to the firmer pedal and that there is like a half pedal that feels like power and then it hits a point where it feels like manual brakes. I was thinking that with the larger bore that there would be less stroke and maybe that would keep it in the power brake feel realm?

[tinsoldier]

Did you properly bench bleed the master ? Not sure if that is a brand new (looks brand new from the pic). All the brand new master cylinder I get has a note saying to you have to bench bleed it or at least comes with 2 lines where you can bleed it in the car before installation of the hardlines.

[Longhorn 70]

I have done drum to disc on the front of my C20. I have a prop valve that looks like the one on your truck. I am told that the lines have to be reversed to work.... Could this be the issue?

Have you called CPP? I am sure they have run into a question like this before.

I have not hooked it up YET. That comes this weekend. I went back to the distribution valve for now and it feels spongy.

[shawno72]

Originally Posted by Longhorn 70

I have done drum to disc on the front of my C20. I have a prop valve that looks like the one on your truck. I am told that the lines have to be reversed to work.... Could this be the issue?

Have you called CPP? I am sure they have run into a question like this before.

I have not hooked it up YET. That comes this weekend. I went back to the distribution valve for now and it feels spongy.

Well I got new pre bent brake lines and from the prop specs the rear and front lines are in the correct position.

[lolife99]

I would try a stock 71-72 M/C.
They have the 1-1/8" bore.
http://www.oreillyauto.com/site/c/de...01292&pt=01292

[shawno72]

Thanks Keith no actually got one of those yesterday was already headed down that path. Plus I have this relationship with Oreillys so they will swap it no questions asked. I will post updates this weekend. Thanks guys and let me know if you have any other suggestions til then
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[Ol Blue K20]

Shawn, after reading the link you posted, I think the larger bore is what you need. I think you're heading down the right path.

[shawno72]

Well the silver lining in all of this is I will be able to pre paint the new master Thinking of just going with a flat clear or some aluminum paint that I already have.

[lolife99]

Originally Posted by shawno72

Well the silver lining in all of this is I will be able to pre paint the new master Thinking of just going with a flat clear or some aluminum paint that I already have.

That's why I will only use aluminum M/C's from now on.
No painting that will eventually rust anyway.
I painted my '69 M/C with cast look caliper paint,... but now it's starting to look like crap.
(with almost no miles on the truck)

[shawno72]

Originally Posted by lolife99

That's why I will only use aluminum M/C's from now on.
No painting that will eventually rust anyway.
I painted my '69 M/C with cast look caliper paint,... but now it's starting to look like crap.
(with almost no miles on the truck)

You know I considered that as well but now I want to make sure I get the right master and for only $14 its hard to beat the steel version. I will swap it once it starts to look crappy...maybe

[jocko]

I believe the discussion above regarding which line from the mc goes to which port on the prop valve is valid - but only for a 3/4 ton truck (pic of my 3/4 attached). Note rear mc res attaches to the front port of the prop valve and vice versa. You can see the 1/4 (rear) lines and figure out which ports/reservoirs feed the rear brakes and which other ones feed the fronts.

My inline tube purchase matched the old lines between mc and prop valve.

BUT, your truck is a 1/2 ton - so my point is dispel the confusion between 1/2 and 3/4 ton. Your mc to prop valve lines are right for a 1/2 ton and the pic below is right for a 3/4 ton.

[BB72CHEVKT]

Originally Posted by lolife99

That's why I will only use aluminum M/C's from now on.
No painting that will eventually rust anyway.
I painted my '69 M/C with cast look caliper paint,... but now it's starting to look like crap.
(with almost no miles on the truck)

Yep, mine rusted before it was done at the shop. I have a aluminum one for my 72.

[shawno72]

Originally Posted by BB72CHEVKT

Yep, mine rusted before it was done at the shop. I have a aluminum one for my 72.

How about some aluminum colored paint

[BB72CHEVKT]

Originally Posted by shawno72

How about some aluminum colored paint

That is what I used and it still rusted.