On my 57 chevy car I replaced the single master cylinder with a dual about a year ago. Ever since having done this the brakes are really spongy, kinda feels like maybe there is air in there, but I've bled them 3 times and still it feels the same. The single master that was on there was fine. Could I be having this problem if the rod on the pedal is too short?
I did this same change over to a 59 mopar, but I added an inline proportioning valve. It stop well and does not pull in any direction. You might have an air bubble in the system too.
also if you went to a larger bore it will feel spongy and more effort to push. a rod depth too short will have more travel (until it hits the piston) and could not fully engage one circuit (which could be called spongy also).
So if I did go to a larger bore is there a way to solve this? I did this same thing on my 60 Falcon and that worked out great.
Drum drum setup or do you have disks in the front? I'd guess it's the master cyl sizes.... when i was doing my truck (drum drum) i started off with a 1" bore and bled the system out and the pedal was rock hard.....swapped to 3/4" and it was spongy.....tried a 7/8" and it was perfect. Using 2 seperate masters one front, and one rear, so can't help you on picking a new master out, but on the old 64 wagon, disks in the front and drums in the rear and used a non power master cyl outta a 70's vette worked great...
I installed a 69 Nova 4 drum brake standard dual master cylinder on my 57 and had to lengthen the pushrod. Don,t ask me how much cause I,m old and my memory is fading.....
besides getting another master, you could change sizes on the wheel cylinders.....i don't think res valves will help at this point. 55-58 chevy masters had a 1" bore what master did you use and is there any play in the pedal before you feel it hit the piston? or on the off chance you could just have a bad master (could be bypassing internally).
I just swapped a dual-chamber unit onto my '39 and had simillar problems. I did have to lengthen my pushrod for the MC. I checked it by getting under the car and feeling the pushrod itself. There was a little play in the rod so I lengthened it until I had taken up all of the play. This helped a little. Because of the PITA method of mounting my MC (in the stock under floor location) I did not bench-bleed my MC first. Even though I am a professional mechanic and do brake work almost daily I still had an air bubble trapped in my system two days after I thought it was done. My advice would be to bleed it 'til there's no more bubbles, then bleed it again, and then when you're sure it's good do it once more. Also it would probably be worth checking to make sure the drums are adjusted correctly (if you are still running drums). Jay
Sometimes there will be a small leak in the brake lines that lets air in and prevents you from doing a good job of bleeding. And when all the fluid eventually leaks out, it gets exciting. But it's more likely there's air trapped in the m/c. Also check to see that there's pedal when you have a bleeder open at one end of the car, do this for front and rear. The whole purpose of the dual cylinder is to give you half of the brakes when a line lets go. If the pedal hits the firewall with one bleeder open, then you haven't gained anything with the swap. Dual cylinders have a longer stroke than single ones, usually there is not enough pedal travel.
The only thing a too short push rod will do is give you a lot of freeplay in the pedal. Eyeball things and see how far the pedal has to move befor it starts moving the piston in the M/C 1/4 -3/8s inch pedal travel is OK. If you still have drum brakes on both ends you need residuial check valves most likely. Most dual cylinders do NOT have them even if they were used with drum brakes both ends, they were in the warning light switch block. Drum brakes do NOT need a proportioning valve unless major changes were made in wheel cylinder sizes. As said earlier you may have a M/C problem internaly. Just because it was new don't maen it is any %^*^$&$^ good !! old partsman saying.But first check the push rod it must have a bit of clearance but not a lot.
Thanks all. It's an all drum setup and I don't remember what master it is, I think from a 68-70 impala. It was a year ago when I put it in. I'll try another master. Anyone have a good suggestion for a 57 chevy? Sounds like I'm looking for a 1" bore.
the master is on the firewall so you won't need res valves. you can use a 67-70 truck master (4 wheel drum of course) or a 67-70 chevy full size car. make sure there are no leaks and the push rod travel is correct. on a side note i used 2 glass jars 1/4 filled with brake fluid and clear tubes to bleed my master on the car ('47 ford master under the floor), same with the new hard lines i made.
I second that. All I do is brakes at Les Schwab. 8 out of 10 times it's the master. I'd start by bleeding the master again, try tapping on it with a hammer while bleeding to break loose any bubbles.
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