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  #1  
Old 05-25-2017, 12:20 AM
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Default Disk brake conversion problem

Hi Guys, so I finished my disk brake conversion using Scarebird brackets and suggested hardware, installed a Dual MC (68 Cougar) and a disk drum combo valve. I updated the drums with new hose, wheel cylinder and shoes. I thoroughly bench bled the MC and vacuum and pump bled the entire system several times. I adjuster the rear drum so they have friction when rotating the tires and redid this after driving a few times.

I have 2 problems:

1. The brakes take allot of force to slow and stop the car - I believe the original KH under dash booster may not be working, so I'm either going to rebuild or replace the KH booster, or switch to a Dual 8" booster under the hood (http://www.ebay.com/itm/8-DUAL-UNIVE...tVB6A8&vxp=mtr). The repair of the KH is allot simpler, will the 8" dual booster give much better performance?

2. the front disk brake engage before the the rear drum brakes, to the point if I don't slightly pump the brake before making a stop I think only the front are doing the stopping. I jacked up one side of the car and had a helper slowly push the pedal down to confirm this, when the front start to lock up to the point where i cannot turn them by hand, the rear are still loose and not engaged. any suggestions I have nothing else to try? can the combination valve (http://www.macsautoparts.com/ford_th...1955-1979.html) be adjusted?

Thanks Boris
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Old 05-25-2017, 08:52 AM
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DANG! I hate it when things don't go as planned (which is all the time for me so I'm almost used to it by now)

Prob #1 - it does sound as if the booster is not working - could also be an adjustment. If I remember those have a trigger valve that determines when the booster kicks in and or the booster rod adjustment. Would like to see how you mounted the master cyl (just for fun).

There should be a check valve on the vac line - make sure it's oriented the right way. Have you checked your vac pressure at the booster? Did you get a chance to drive the car with drum brakes and was the booster working then?

As for preformance I'm sure the dual action booster would be "better" than the KH - "much" better - maybe - maybe not. I'm using the OEM under hood booster with a Lincoln master cyl. It's about like a newer 90's full size van to stop not hard but not a light pedal either. I don't think you could have the KH booster rebuilt for the $54 price of the dual action booster. Booster Dewy who did mine charges $155 for the Bendix rebuild and $175 for the KH. Maybe there is a cheap kit available now. I understand though. If you go with the under hood booster you have to find mounting brackets and change all the plumbing etc - that's time and money.

Looked at the '68 Cougar master cyl. I know this is a dumb question but I see two types available - disc/drum and drum/drum. You are using the disc/drum master cyl correct?

If so I'm guessing you double checked the plumbing to make sure the small reservoir on the disc/drum master cyl is connected to the "rear brake" port on the combo valve and then to the rear line only?

Which leads to I guess you separated the system into two separate parts - front and back?

Not sure what the results would be if using the drum/drum master....?

Prob #2 - I'm using that same combo valve. Believe my front brakes engage first also but only momentarily - the front end doesn't dive in a panic stop so I didn't worry about it too much. My rear brakes are definitely working though - not like yours. Almost sounds like you have it plumbed wrong where the residual valve is not in the rear circuit????

There are a lot of great folks on here to help - we'll get it figured out.......

Eric

This is a pic of when I installed mine 12+ years ago. Had planned on putting the combo valve under the master cyl but guessed it would be too close to the manifold so stuck it down on the frame (thus the plumbing mess). Worked so well I never have bothered to go back and clean it up.


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Old 05-25-2017, 08:57 AM
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Boris, two truths are at play here. Drum brakes require much less boost to operate because the 'forward' or primary shoes actually help apply pressure against the drums. Disk brakes don't have that configuration so they require MUCH more boost. That means, the power booster that came with your car is far too small to make any difference on calipers. I suggest using the largest booster you can fit, which is the dual-diaphragm 8".

I see the combination valve offered and you say it is for disk/drum but I don't see that at all in the listing. Because it simply doesn't say, this valve could be for a disk/disk setup. The front ports are open to the M/C on these valves meaning, the disk calipers get full pressure at all times. I believe you aren't developing enough pressure from your OEM booster to run the combination valve properly.

Some of our members left their K-H booster intact because they believed it helped with 'some' boost. Other members disconnected the vacuum and found the booster made NO difference what so ever. That part is your call but you need a much larger power booster for disk brakes.

BTW, what size piston is in your M/C? - Dave
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Old 05-25-2017, 09:10 AM
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Dave - I see what you mean on that combo valve. It says for disc brake systems but not "front" disc brake systems. Isn't that round module on the left side of the valve in the pic the residual valve though? (which would only be used on rear drum front disc systems?) Probably just not enough info on MAC's site but you are right - need to know for sure that it is disc/drum.

edit....

I saw a chat option on MAC's so I asked about the valve....

Me.........Re: 66-73874-1 the disc brake valve. Is that for disc/drum systems or disc/disc?
MAC's.....Let me check that.
MAC's.....It can be used for either.


NOPE!




Think I found my answer on the master cyl if using one that has two of the same sized fluid chambers - the valve does the balancing work but you have to be sure and plumb the rear port on the combo valve to the primary port on the master cyl as in this pic.

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Old 05-25-2017, 09:12 AM
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My 59 underdash booster was working fine.
Sure did not feel like it was doing anything though.
It's really not even big enough for the drum brakes, IMO.
I permanently it to install my custom firewall bracket

As for rear drums, how tight do you have them adjusted?
That can make a huge difference on how soon they start
to brake in relation to front disk.

I finally got my setup on the road and the front will lock
much sooner than the rears. In my case, my booster, MC
and front caliper piston size are substantially upgraded
vs the stock rear drums (and even over what most on
this board upgrade their disk setup too).

That said, I have no issue with locking brakes with minimal
foot pressure. My pedal travel is longer than I thought it
might be, but it makes the brakes very progressive.

I personally feel the dual 8" that most here use is the bare
minimum for a SQ disk conversion.
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Old 05-25-2017, 10:19 AM
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Quote:
Originally Posted by OX1 View Post
My 59 underdash booster was working fine. Sure did not feel like it was doing anything though. It's really not even big enough for the drum brakes, IMO.
OEM brakes were barely minimal at best. Seeing a trailer ball on a SB bumper makes my skin crawl with the OEM braking system.

Quote:
Originally Posted by OX1 View Post
I personally feel the dual 8" that most here use is the bare minimum for a SQ disk conversion.
I agree, but that is the largest booster we can practically install.

Quote:
Originally Posted by DKheld View Post
Dave - I see what you mean on that combo valve. It says for disc brake systems but not "front" disc brake systems. Isn't that round module on the left side of the valve in the pic the residual valve though?..

edit....

I saw a chat option on MAC's so I asked about the valve....

Me.........Re: 66-73874-1 the disc brake valve. Is that for disc/drum systems or disc/disc?
MAC's.....Let me check that.
MAC's.....It can be used for either.

NOPE!

Think I found my answer on the master cyl if using one that has two of the same sized fluid chambers - the valve does the balancing work but you have to be sure and plumb the rear port on the combo valve to the primary port on the master cyl as in this pic...
Eric, don't you just love it when vendors give out wrong advice? In this case, that valve information may be dangerous. Some vendors simply remove the spring on the 'rear system' of the valve. That stops all pressure reducing features, then used for Disk/Disk operation. The valve's piston is still retained as a mechanical balance between both hydraulic systems. Because caliper pads are always very close to the rotors, it is important to keep rear shoes close to the drums as well, either by using a working self-adjusting system or by installing a 10-lb residual valve. Combination valves DO NOT include a residual valve.

Because the Corvette-type aftermarket master cylinder has the same size pistons & reservoirs and no internal valve, it really doesn't matter which side you use for front or rear plumbing. - Dave
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Old 05-25-2017, 01:26 PM
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Quote:
Originally Posted by simplyconnected View Post
OEM brakes were barely minimal at best. Seeing a trailer ball on a SB bumper makes my skin crawl with the OEM braking system.
Wonder if my parents have any pics of their 60 SB towing
our 59 Woody Thompson boat. Usually, dad used our 65 ford
wagon, but that was down and out one year, so he put a
hitch on the bird.

Only a 17'er with 55 HP Homelite, 4 cycle, OB (and like the bird,
he still has that outboard somewhere in the basement), but
pretty heavy for that size.
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Old 05-26-2017, 01:52 AM
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That wooden Thompson boat was light, like a cork in the water. My Dad had one as well. We got it up to 42 but couldn't squeeze any more out of that 75HP Johnson. Tons of fun...

Even so, trailering behind a Squarebird is a scary thing. The body and engine were plenty big enough but the brakes were wicked-horrible without disks. - Dave
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Old 06-04-2017, 06:33 PM
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Thanks for all the comments.

Looked further into the combination valve and it is a Disk/Drum.

The Master Cylinder is. 1" bore.

I've ordered residual valves for the front and back.

My car has factory AC.

My booster below the dash does seem to work(pedal drops when motor is started) but the pedal comes back up very slow so I think it has a problem with the valve that lets air back in the booster, so I'm going to replace it.
I've ordered a 8" dual booster and some linkages, but I'm not sure about the bracket and offset linkages. Does anyone know of a bracket/linkage setup that is a bolton solution, or of some drawings how to build this?
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Old 06-04-2017, 09:05 PM
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Default Disk brake conversion problem

Boris, ABS should have the dual MC/8"dual power booster AND the mounting bracket that is made to clear that AC plenum box. Check with them. They will NOT sell the mounting bracket by itself...
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