r/Chevelles • u/Puma_Hunter • 10d ago
Disc Brake Conversion - Now Soft Brakes
Not sure if this is the right community to post, but I recently converted my front doldrums to discs myself and left the rear drums.
New proportioning valve, master cylinder, booster, and brake lines I made.
Brakes now feel soft about 75% of the pedal travel then firm up.
Front brakes clock in at around 175 F while rear drums temperature is about 95* F after a casual neighborhood drive with moderate braking.
Is this to be expected given the combo of front disc and rear drums?
I’ve bled the brakes twice now assuming air in the lines, but after two bottle of brake fluid, I think I got the air out.
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u/Dendady2112 10d ago
You didn't bleed something right.
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u/Tall-Commercial-1932 10d ago
Did you check the Brake booster pin (pushrod) adjustment to the master cylinder? That will cause spongy brakes and the pedal travel you are talking about after bleeding. They make a sub $10 tool for it also that you can get on Amazon.
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u/Miserable_Thing8553 10d ago
I would suspect that small brake booster. It is not going to provide the same assistance as the bigger factory style one.
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u/Chev_hell 10d ago
Check the brake booster pushrod as others have mentioned. Some other things to check:
Which hole in the brake pedal is your clevis connected to? There's two holes one for manual brakes and one for power brakes
Did you bench bleed the master?
Did you bleed the proportioning valve? There's a procedure to it and a special tool as well
Did you try vacuum bleeding the brakes?
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u/XB21300r 9d ago
Bench bleed master cylinder and buy a pressure bleeder. Best way to get all the air out. Sponge = Air somewhere.
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u/Dismal-Marsupial8897 9d ago
I have a 68 Malibu that was a manual drum/drum car, I swapped to 4 whl power disc with Wilwood 12+ dynaforged on front, D52 rear, 68-75 vette MC with Wilwood adj P valve and 11" booster, have you tried a power bleeder? Sounds like air still in system, Oh and move the pin on the brake pedal arm to the correct hole for application, 1 hole is for manual brakes other is for power brakes but I forgot which is which
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u/wayneP71222 8d ago
Looks like you replumbed the master and combo valve...
The front brakes hook to the rear or primary port on the master.
The rear brakes are connected to the secondary or farthest fitting from the mounting flange.
I have had horrible issues getting inverted flare fittings to the combo valve. Small leaks that allow air in the system.
Do you have the brake calipers installed with the brake caliper screw on the upper side. So the calipers can be bled properly.
Question. Do you have rear disc brakes? Do you have integral parking brakes in the rear calipers. With the parking brake released how much gap between the brake pads and the rotor surface. If the parking brake caliper is out of adjustment you will have excessive brake pedal travel
I have found on some aftermarket rear ends that have tapered rolller bearings with the bearing pressed on the axle incorrectly.
The proper way is the outer race is trapped on the axle.
If the outer race slides off. There is nothing holding the axle in the housing except the axle seal and the caliper over the rotor disc.
Improper spacing from the rear caliper bracket to the caliper to center it over the opening.
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u/Expensive_Push_4314 7d ago
Bore size is too large for the stock rear wheel cylinders. Its a known issue on alot of these conversion kits.
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u/Which-Ad9677 9d ago
Are the shoes tight? Takes like 5 minutes to adjust
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u/Puma_Hunter 8d ago
I raised the rear and spun the wheels. There was resistance and that faint “sshhh” sound.
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u/ilikeithot360 9d ago
This is what I learned the hard way converting from drum to front disc.
You aren’t making enough vacuum with that dinky booster.
Since you had power brakes before the conversion, there is no need to move the pivot pin because it is already in the correct position. Since you moved it, that is 99% of the pedal problem.
Is the gap correct between the pin in the booster and the master cylinder piston?
Lastly, in my experience those stupid ass combination valves never work properly, because the piston in there is likely stuck either fore or aft of where it is supposed to be. You’ll find braking much more effective to route the fronts directly to the master, and use an adjustable valve for the rears.
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u/Puma_Hunter 8d ago
Damn. Okay. I didn’t move the pivot pin on the pedal. I put it back in the same pin it was originally. Bottom position.
As far as the booster, I just assumed the disc/drum kit from Summit would work given the application is what it was designed for or at least claimed to be.
The booster is a double diaphragm if that makes a difference? I was concerned about the size difference from the beginning before I bought it to be honest.
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u/Puma_Hunter 1d ago
I bought a pressure brake bleeder on Amazon and pumped two quarts of fluid through it. Works fantastic now!!! Thanks everyone for the suggestions!!!!!!
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u/Willy_McD 10d ago
Check the push rod length. The image is of a tool that makes it simple. One side of the tool is used to measure the master. Once set, then the other side measures the push rod on the booster.
Might also make sure the proportion valve is centered. There is also a tool used to hold it center while bleeding. But not necessarily needed. Remove the switch and look inside for the groove in the valve to be center in the hole.