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 Post subject: rear disc conversion
 Post Posted: Sun Mar 13, 2011 5:28 pm 
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am looking at fitting rear disc instead of the drums on my 9 inch,, i have a manual master cylinder 1.125 inch.. i think this is an improvement but have been told that i will not get enough brake line pressure to emergency stop the car,, is this true and would i be better off keeping the drums on the rear end,, i have 4 piston calipers in the front and single piston sliding calipers on the rear


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 Post subject: Re: rear disc conversion
 Post Posted: Sun Mar 13, 2011 6:19 pm 
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So does it stop OK now with the disc brakes on the front and drums on the rear? If it does, then what would be so different about disk brakes on the rear? :-k You ARE running a dual master cylinder, right? :-k

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 Post subject: Re: rear disc conversion
 Post Posted: Sun Mar 13, 2011 8:23 pm 
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We got the economy disc bake kit from Moser for their 12 bolts. They also have them for 9". It's around $300 complete except brake hoses. We got the manual brake master cylinder from a 70 vette. They had 4 wheel disc brakes. That was like $40 new from Advance Auto. Jake said it stops in almost half the time the rear drums did. The weight was about the same and the wheels stick out about a 1/8 to 1/4 inch more. cause of the thickness of the rotor hat.

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 Post subject: Re: rear disc conversion
 Post Posted: Mon Mar 14, 2011 1:17 pm 
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ken,, its never stopped in a confidence inspiring way and i have decided at last to get it sorted,, got the disc on the axle now,, have ordered a brake pressure gauge to check the pressures when its completed,, but am concerned that the line pressure may be a problem


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 Post subject: Re: rear disc conversion
 Post Posted: Mon Mar 14, 2011 5:13 pm 
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zephyrc wrote:
but am concerned that the line pressure may be a problem


There's ways to get around that to a point. Changing the ratio on the brake linkage can give you more pressure for XX amount of foot pressure but to be honest, manually operated disc brakes on a heavy street car won't ever be as easy to stop as power assist brakes are.

There were several American made mussel cars that could not be sold with power assist disc brakes because of a lack of engine vacuum. If memory serves, the 1969 Z28 Camaro with the optional off road cam was sold with disc brakes but did not have a vacuum assist booster.

Smaller diameter piston in the master cylinder will give more line pressure per pounds of petal pressure but you'll be stuck with long petal travel to do that. Changing the ratio on the rod will do the same thing with a larger diameter MC with pretty much the same down side. Dual piston calipers will require more fluid to make them work, which is also a negative as far as small diameter MCs are concerned.

There's a balance in there somewhere though and you just might have to play around with different combos to get where you need to go.

Oh, and if you've got brakes you have to pump twice to make them work then you might want to put residual pressure valves in the lines to keep the linings closer to the rotor.

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 Post subject: Re: rear disc conversion
 Post Posted: Sat Mar 19, 2011 6:48 am 
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I had the same uninspiring braking in the vega,and when i switched the drums out for the wilwood discs in the rear it was a BIG difference...nice to have when some idiot pulls out in front of you at 35 mph O:)


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 Post subject: Re: rear disc conversion
 Post Posted: Sat Mar 19, 2011 11:06 am 
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disc etc are all on now but i cannot test it on the road yet because my axle shafts have severely twisted,, so i am waiting for new shafts to arrive,, should be here in 3 weeks time


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 Post subject: Re: rear disc conversion
 Post Posted: Sun May 29, 2011 10:08 am 
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got the shafts in now and test drove it.. it is a horror..i put disc on the rear and also shanged the master cyl to a wilwood 7/8 inch.. the pedal travel is very long and does not stop the car,, slows it down somewhat but i doubt you could lock the brakes up no matter how hard you tried... noticed when bleeding the rears thateven with all the air out the pedal was still not good.. if i clamp off the rear lines the pedal is fantastic ,, bled it several times and no change.. the rear calipers i used were from a car with abs and they have an integrated parking brake mechanism and therefore the pistons wind out,, would that make a difference


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 Post subject: Re: rear disc conversion
 Post Posted: Sun May 29, 2011 12:42 pm 
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So what kind of car are the calipers off of? Most of the older style rear calipers will not operate properly without the park brake hooked up and you actually had to use the park brake to get them to adjust. Some calipers will interchange from side to side but it puts the bleeder at the bottom and of course you can't bleed the air out of them like that. The only other thing that comes to mind is to make sure the tab on the back side of the pad aligns with a notch in the piston. Sometimes you have to rotate the piston slightly to get it lined up.

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 Post subject: Re: rear disc conversion
 Post Posted: Sun May 29, 2011 1:40 pm 
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WHERE ARE THE BLEED VALVES FOR THESE CALIPERS??? IF THEY ARE ON THE BOTTOM, YOU WILL NEVER GET THE AIR OUT OF THEM WITHOUT TAKING THEM OFF AND TURNING THEM WITH THE BLEED VALVE ON TOP.

I know a local guy put a set on upside down back years ago and went through about a gallon of brake fluid before he asked me what was wrong. Soon as I looked at it was plain as the nose on your face. He took them off, turned the bleed valve up and put a piece of steel in between the pads then bled them and they worked fine after he reinstalled them.

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 Post subject: Re: rear disc conversion
 Post Posted: Sun May 29, 2011 5:32 pm 
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the calipers are of an english ford,, a mk3 granada 1993,,, bleed screws are at the top,,, i dont have the parking brake hooked up at this time, need to get the cables made.. will check the tab on the pads is in the groove on the piston......... thanks for the help fellas


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 Post subject: Re: rear disc conversion
 Post Posted: Sun Jun 05, 2011 3:44 pm 
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put the original master cyl back on ,,its a strange unit with a larger bore than the wilwood and the brakes felt fine whilst driving on the street,, but went to the track today and had an interesting moment at 140 mph, were although the brakes felt good and the pressure seemed fine the car only barely stopped ,, and i do mean barely... it took almost quarter of mile to stop whilst applying reasonable amount of pressure


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 Post subject: Re: rear disc conversion
 Post Posted: Sun Jun 05, 2011 7:32 pm 
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What type/brand brake pads do you have on it? Are you sure they aren't contaminated? I had a friend that was having issues and I figured out that they had gotten brake fluid on the rotors during bleeding. That fluid found it's way to the pads and just a small amount of that will make the brakes slippery. New pads solved his problem. If it's available get a more aggressive pad.

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 Post subject: Re: rear disc conversion
 Post Posted: Mon Jun 06, 2011 3:05 pm 
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gearhead,, thats a very good idea.. the pads were on the car when the brakes were bled so there is a very good chance fluid got on them,,, will order new ones in the morning and then try it on the brake tester


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 Post subject: Re: rear disc conversion
 Post Posted: Mon Jun 06, 2011 6:54 pm 
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FYI, my Studebaker street car weighs in @ 3459lbs with me in it and stopping it @ 137 wasn't fun even with 9" disc fronts and 11" drum rears. I burnt up a couple of sets of front pads on those 9" rotors before changing to something that worked a little better, ie, if memory serves, those linings are now metallic instead of organic. With the organic linings on quarter mile passes the front brakes would be smoking when I got back to the pits.

140MPH is approaching if not already at a speed that a chute should be used especially on a heavy car. That might be a worth while investment some time in the future.

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