My neighbor recently bought a 40 Ford Deluxe coupe that was built in the 90's. It has been converted to front manual disc brakes on a dropped axle, the guy he got it from also bought it that way so he doesn't know what components were used. The brakes have never been right as the pedal is soft despite bleeding over and over. The PO tried different master cylinders, the one on it now is off an 80's Camaro with a 1 1/4 inch bore. It has a proportion valve that is not adjustable. I have searched the H.A.M.B. but so far with no luck to tell him what to try next other than find a master cyl. with a smaller bore, like 1 inch. I have driven the car and he is right, it gets scary trying to get it stopped. He is going nuts as he wants to drive it as a DD a lot but he is afraid to. Anyone got any ideas?
I've had air stuck at the master cylinder before that wouldn't bleed. Try bleeding right at the lines on the master cyl.
I have used 1" bore master cylinders almost exclusively, with Disc/drum combinations, with modernish brakes, ie 70's stuff, GM and Ford, used with proportion valve. Important to remember disc needs about 1400psi to operate effectively, drum needs about 500psi to operate effectively, this can all be calculated based on cylinder sizes, from memory the smaller the master the more pressure less volume, the smaller the slave the less pressure. Pedal ratio is also important if not using a booster, a ratio of around 7:1 I have found to be effective. http://86.43.94.97/moodlecp9a/mod/resource/view.php?id=256
Any chance he has "low drag" GM calipers? See how far the back off when brakes are released. Been discussion on similar "soft" pedal problem concerning these calipers here recently. 1-1/4" bore master is normally too big for manual brakes.
I have a 40 with Power brakes, disk/drum. I used a 1inch bore vette style MS and a 7in single booster with a adjustable proportioning valve and residual valves 2LB/ 10LB for front and rear. I had problems bleeding as well. Here is what I did. Check peddle ratio and slack in the link to the booster Vacuum bleed all four corners Manuel bleed all 4 corners Lastly, power bleed through master cylinder......should have done that first. can do this with an old modified Master cap. My master is in the stock position under the drivers floor board and about level with the wheel/calipers. Its a pain in rear to get all the air out. Brakes are good now.
There was a recent thread on here that showed that if the calipers weren't on the correct sides of the car that there was a pocket of air trapped even if the bleeders were pointing up. Hopefully someone can post a link to that thread.
One thing no one has mentioned is Dot 5 synthetic brake fluid. I have tried to use that fluid, and we bled the brakes forever trying to resolve a soft pedal. On a recommendation, I drained, flushed, and installed Dot 4 fluid and presto. The spongy pedal was resolved.
https://www.jalopyjournal.com/forum/threads/brake-calipers-on-wrong-side.1162304/#post-13727997 Found it!
When I got my '40 it had a barely functional, poorly executed front disc brake conversion. To fix it I installed a Maverick manual master cylinder, I believe it uses a 15/16 bore. I had to use an adapter and extended the pushrod about half an inch. I also installed residual valves and a Wilwood distribution/proportion block with brake light switch provision. I split the front lines at the distribution block and did away with the factory tee at the driver front. Stops great now.
The master cylinder might be too large. The area of the bore is inversely related to the force for pressure. (P = F/A). A smaller diameter bore would help build pressure with a downside of longer brake depression. Maybe add residual valve too so the drums already have preemptive pressure. Albert
The Jalopy Journal
