No pedal pressure after front disc swap?

Ok, I've searched and searched, now it's time to stump the experts. None of the other disc brake questions seemed to fit my situation.

So I have just completed a front disc swap with a thin disc setup from a narrow track dana 30. New calipers, rotors turned, hubs machined, residual pressure valve removed from the stock '74 master cylinder, etc.... After bleeding a large bottle of brake fluid through the front brakes there is no pedal pressure, pedal goes all the way to the floor, even after pumping the pedal as much as a dozen times.

The brake system worked fine prior to the swap, now.............nothing. Rear drums are adjusted properly, master cylinder had no issues prior the swap, no leaks that I can find.

Another thread brought up the possibility of mistakenly using a thick disc caliper, but will a thick disc caliper mount to a thin disc caliper bracket? There didn't seem to be enough room between the pads and the rotor to fit on the larger rotor. I am thinking that this is not the issue.

Any thoughts? Thanks in adavance.

 

I'd suspect the master cylinder. If the pedal travelled further than normal during bleeding, built up debris in the master cylinder could have gotten between the cylinder bore and seal. seen it plenty of times after brake jobs on older or high mileage vehicles. Might check and see if the m/c itself is building pressure.


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Another oft seen problem when doing a DB swap-are the calipers on the right side?

 

..I agree..air bleed should at highest point..

 

Also make sure you removed the 10psi check valve out of the master cylinder.

 

Thanks guys, I removed the check valve and made sure the bleeders are on the top.

I'll check the master for pressure tonight. It's probably about time to change it, as they only last 5-6 years for some reason. Its been about that long but I can't remember for sure.

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the fluid collects moisture and other goop thats why you are supposed to change it .... but its not on the top of the list.

 

Yeah I know, but like you said it's not on the top of the list.

 

Ok, so I disconnected the port to the front brakes and plugged it. The master cylinder holds pressure just as you would expect. What's next? Both bleeders are up and the residual valve is removed. How do I test the calipers? Since there is nothing in between but hard line and new flex hoses, my next thought is the calipers are bad. How do I go about checking? remove a brake pad a see if the piston moves? Could they be stuck, they are "new" rebuilds.

 

if you pull the caliper with nothing for it to push agains won't the brakes explode ..? have the kid hit the brakes and see if they move .could be alot of trapped air know anyone with a power bleeder?

 

Could be a stuck caliper piston. Jack up the front and try to spin the wheel while the brake is applied and see if it holds.

Of course that doesn't necessarily answer the pedal to the floor issue.

 

There's so little to a caliper there's only two things they can do-seize or leak. if they were seized you have a hard pedal with no braking. If they were leaking, well, that's pretty much self explanatory. Picture of your caliper installed?

 

Even disks perform better with some residual pressure. I'm running willwood 2psi residual pressure valves on from front disk conversion. It helps keep the pistons from retracting too far so you don't waste your pedal travel pumping them back up every time you hit the brakes.

http://www.speedwaymotors.com/Wilwood-Blue-Disc-Brake-Residual-Valve-2-PSI,1377.html

 

Not really needed on in intermediate CJ were the mc is the highest point in the system, on an early with a frame mounted mc absolutely.

I suspect it's a bandaid for having a incorrect master cylinder. The bore size for a disk/drum mc is different than a drum/drum unit.

 

I've got a under floor MC bacause of the PO's swap of a 65 body on to a 72 chassis, he used the frame mount set up for the MC.

 

I'll double check the caliper movement tonight and get some pics up.

The bore difference between the drum/drum and drum/disc master cylinders is 1/16" according to the specs.

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that sixteenth multiplied by stroke could be it...just a question you have the larger res going to the front right?

 

I am using the stock 74 master, which is supposed to be doable. The reservoirs are the same size, and have double and triple checked to make sure it is connected to the correct port.

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The 74 has proportioning valve correct? I believe there is a procedure to bleed that.

The stock mc will "work" but using a disc brake master with the proper bore size will improve performance.

I wasnt happy till I switched to a late 80's waggy booster and master cylinder.

 

It has a distribution block, I am not sure if it is also a proportioning valve. The original drums have 2 different size wheel cylinders.

Anyway I enlisted my auto mechanic neighbor to help me bleed it out. BUTT LOAD of air in the master cylinder. Bled the whole system and went back to he the master, AIR AGAIN!! Looks like it's time for a new master. Might as well go with the disc version. Oh and checked the calipers for movement, ok there.

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