i have a 1951 mercury with 4 wheel drum brakes. i have installed a duel master cylinder with a power booster i have kept the floor pedal. i am using dot 5 brake fluid the car stops good but i don,t like the pedal feel.. it,s kind of soft i know there is no air in the system .i have been told that it,s because of the dot five. has any one had this problem & if yes what is the best way to flush the system & use dot 3 which is said will not compress.thus giving me a harder padel
Fluid is fluid.....it does not compress, either you have air in the system, or a vacuum leak.....as suggested pedal out of adjustment.
Dot five is actually slightly more compressible than glycol based fluids and can cause a slightly spongy pedal, it is also more prone to being aerated. Sent from my iPhone using H.A.M.B.
A manual underfloor peddle has a different ratio then a power master cylinder. All the air out of the system? Shoes adjusted correctly? All play removed from the brake pedal and any pushrods and levers ( don’t forget the one from the booster to master) I’m not a fan of dot 5 , I would rather use super dot 4 fluid. But dot 5 is good fluid and you could put water in a system and bleed it and get a good pedal until the water heats up and turns to steam. Any fluid is non compressible.or a very minor amount of compressibility under large amounts of pressure applied.
To flush the system, just run enough fluid out that you push all the dot 5 out and start seeing the dot 4 fluid coming out of the bleeders. Usually 3-4 pints.
Manual to power, with the same pedal ratio and master cylinder bore size and wheel cylinder bore size, you’re going to have a very easy pedal. By “soft”, do you mean that the pedal sinks under foot pressure? Or that it just feels too easy? Check the clearance between pushrod and master cylinder? Too little and you drag the brakes. Too much and the pedal feels soft, but doesn’t sink to the floor. Sent from my iPad using The H.A.M.B. mobile app
Fact is silicon fluid is a little squishy, if everything else is right; you just need to get used to the feel.
i spelled solid wrong & some people just cant let it go.it must be nice to be perfect. to those that helped thank you
You should change all the rubber parts (wheel cylinder and dual master cups, hoses, residual valves) when switching to or from DOT 3, 4 or 5.1, along with a good denatured/isopropyl alcohol flush. As already stated, your manual pedal ratio is too much for most boosted systems, especially with drum/drums.
If I was changing over fluid types, for the little that masters and wheel cyls cost these days, might be just as easy to get new ones. On the issue raised on pedal ratio; under floor is probably one of the baby sized 7" booster; they haven't seemed to be overly sensitive with a 6:1 ratio in the couple of units we put together; but I can imagine if you had a large OEM sized booster it would.
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