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Disk brake conversion problems! Ideas helpful.

Started by 62 driver, May 21, 2022, 01:46:40 PM

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62 driver

I did a disc brake conversion on my son's 61 Cadillac. It turned out good and it stops on a dime. It was so good I decide to do it on my 62 Cadillac. But unfortunately it didn't go the way the 61. My disc brakes and rear drum brakes barely stop the car now. The booster has been rebuilt along with the master cylinder by Karps Brakes eight years ago. I removed the restriction valve for the front brakes. I put in 3/16 brake lines to the two front brakes leaving the back brakes and brake lines stock. I replaced all four rubber brake hoses. I just swapped out my master cylinder with a good one from my son's car. That did not change or help matters. My problem is you step on the brake pedal with the engine off and there's hardly no movement, you start the engine and step on the brake pedal it goes down about an inch and a half from that point on it's very hard to push the brake pedal to make the car stop. I bled them very well. The rotors are clean The pads are new. You cannot lock up the brakes the way they are now. I'm at my wits end and I don't know what to do next. Any ideas would be helpful. Thanks Dave
Dave Schneider,  CLC #27889

fishnjim

Sounds like to me, you increased the front brake line size and that's what's killing you.   Takes too much fluid volume and the master only puts out so much per stroke.   The proportioning valve transfers some of the fluid to the rear to increase the % rear braking.   So if you negate that, the rears more ineffective.
There's ways to balance all that, but you don't give enough info of what parts, you used, etc.   A good reasons to buy a kit rather than DIY.
I'd go back to stock lines and reinstitute the valve and see where you are, then go from there.   Make sure the new hoses are the right size too.
I don't think you'll win any braking contests with a car that big, but safety first.  It has to stop in a normal distance.   Rearender is always at fault.   
The master has to match the wheel cylinders.   If the '61 is smaller, you need to change that as well or get an after market that's sized right for the disc brakes.   If you look at the modern brakes, they use smaller lines (metric), more pistons, and keep the fluid volume small.   In hydraulics, it's the ratio of the diameters that determines applied force.   The pressures the same in all directions in fluids.
for better response.

62 driver

Thank you for your response. I had the original lines running to the two front disc brakes and I was having the same issue as I have now. Someone recommended that I use the smaller lines (3/16 not 3/8 sorry). like you just did.  I replaced the Original with 3/16 inch lines. The set up is identical to what my son has and this is a kit except for the booster and master cylinder. I also have a manual proportionate valve as well going to the rear drums.
Dave Schneider,  CLC #27889

Dave Shepherd

I have done many of these conversions in my shop,  no Cads though. Never used anything but 3/16 lines. Now, master cylinder bore size comes into play here. Fishn Jim is correct on his advice. You may have a booster issue, see if the pedal presure vs stopping power is the same with the booster vacuum blocked off, make sure also you have good vacuum to the booster first, before trying this.

TJ Hopland

Are you using the same caliper bore size as your sons car?  And you did try his MC?

Are you sure the booster and MC combo is correct as far as the travel?  Strange things can happen if the MC isn't able to fully retract.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

62 driver

Yes I did swap M/C and nothing changed.  I just closed the proportion valve and that did not change the stopping distance. I am going to check out the push rod between the M/C and booster.  That might be the problem because I had to push the master cylinder in to start the nuts. Thanks to all ideas and will report back. Dave
Dave Schneider,  CLC #27889

The Tassie Devil(le)

There should be no pressure on the Master Cylinder when attaching it to the Brake Booster.   If there is, then you are pushing the Brake Piston inwards, and therefore not allowing it to return to the rest position to accept fresh fluid from the reservoir which it needs to do to maintain the correct capacity in the lines.

Plus, it will be maintaining pressure on the brakes themselves.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

62 driver

Thanks to everyone that giving me the help working on my brakes. A good update. I loosen the master cylinder and reset the master cylinder to booster rod. The master cylinder now bolts on without pressure from me. I also closed off the proportionate valve and took the car out for a spin. Now the front left brake locks up the right does not. There's no pulling and the car stopped straight but the right wheel does not lock up. I then open the proportion valve a little bit went back down the road at 30 miles an hour and slammed on the brakes it's slow down but not fast enough still in my opinion. The left brake locked up at about 10 MPH instead of at 20 MPH. Any ideas on why the left brake locks up and the right does not. I'm gonna re-bleed both brakes in a few days to see if that might've been the problem. Thanks
Dave Schneider,  CLC #27889

TJ Hopland

I was going to ask how the pedal feels now.   Bleeding likely would not hurt.   Are the bleed ports at the very top of your calipers the way they are mounted?  If not that could be a problem and you may have to dismount them so they can be oriented with the port at the top for bleeding.

Does your prop valve have a spot for a brake light?  If so bleed that as well as pretty much every fitting you can access.  With aftermarket stuff that is somewhat universal fit there are design flaws as well as just not being able to install things in the optimum position.     
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

49 Caddy

  Sounds like a push rod issue to me.  I am replacing front brakes to disc & adding dual brake master cylinder from Wilwood for my 49 Series 61.  They specifically say how to adjust the push rod & removing parts for certain combos.  (Disc/drum)

Dave Shepherd

May not be the issue, but disc brakes need to be " bedded" into the rotors properly to get effective braking. Check into the procedure online.

62 driver

Thanks to all. So far the problem is about the same. I completely bled all brakes with a pressure bleeder. The calipers are installed correctly with the bleeds on the upside. The brake pedal reacts normally with or without the engine on. You cannot pump up the brake pedal. Going down the road 60 MPH and stomping on the brakes, locks up the left front tire but the right tire does not lock up. The stopping distance of the vehicle is still too long. The brakes are not engaging fully somehow. I am in the process of replacing the booster. I'll report back when I have that completed.
Dave Schneider,  CLC #27889

TJ Hopland

I was just asking about the position of the bleeders since this isn't a factory setup they may not be at the absolute top of the calipers.  Have you tried bleeding old school 2 person style?  Pressure bleeding isn't magic, sometimes it takes some force to work the air out.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason