Have a 40 truck and installed frame mounted brake mc with booster. Brakes do not work well pedal goes to the floor. I.m hearing to install residual valves. Does that usually solve the problem? Where should I install them? Thanks trucke
That's not likely to be the problem. Is this something that you recently put together? If so, you might want to give some more details, like what brakes it has front and rear, how the pedal is set up (especially the ratio of pedal length to pushrod/pivot distance), etc.
What is the bore size on the master cylinder? Is the master cylinder below the calipers or wheel cylinders?
regardless of bore size,- bench bleed, are all lines locked and tight, are your calipers facing the right direction as in the bleeders. any leaks anywhere? are the rear brakes drum? are they adjusted out also? but is the rod the correct length is it adjustable? Do you have power booster ahead of the master? is that little rid in the booster correctly adjusted? I usually have a prop valve. but there are many variables.
Try bleeding the brakes again . IMO residuals aren't needed . Pedal to floor is generally air in the system , or possibly master master bypassing . When you go to the doctor , does he have to play " twenty questions" with you , how about your mechanic ?
The old single piston master cylinders have a "check valve" at the outlet end of the master cylinder that maintains pressure in the line when the pedal isn't being pushed. Firewall mounted dual master cylinders have a small check valve behind the pressed in brass seat of the Drum brake side of the master cylinder none on the disk brake side. The ones on a drum/drum master cylinder will work under the floor with the two check valves if the rig is driven every day or at least several times a week but will leak down if the rig sits for over a week at a time. That is my experience from running one for 15 years. 54 car drums on the front 74 Nova drums on the rear. If you have ever rebuilt a dual master cylinder the check valves and springs come in the kit for drum brakes. I don't recommend attempting to rebuild one though even though I have done hundreds of brake jobs in my life. This being the kit for the 66 Mustang Fruit Jar master cylinder that was the go to hot rod/T bucket master cylinder for years. Note that it has that check valve. I would install the residual valves in the lines close to the master cylinder and go from there. Still a lot of getting brakes right is technique. That includes having a helper who listens to what you say when you have them pump the brakes and does it exactly as they should. That and don't listen to the spit and whittle club's half baked ideas about bleeding brakes that have no chance of working right. I don't know what some of them smoke but I think some of their comments are just to blow smoke up peoples asses and keep them confused.
Need a LOT more information here. Exactly what master cylinder do you have? OEM something or aftermarket something else? What’s the bore and stroke on the master? Pedal length and ratio? Can the pedal fully stroke the master before it hits the floor? What front brakes? Disk or drum? Which ones are they? Part numbers for OEM or aftermarket? Same for the rears, what are they? What else is installed? Got a proportioning valve? Residual pressure valves appropriate to whatever the brakes are at front and rear? Was this all in a kit from somewhere? Or is it assembled from off the shelf parts? If not a kit, did someone research and prove that this combination of stuff should work? On your truck or at least a similar one? Can we assume that you’ve done brakes before and know how to bench bleed the master? Bleed the system? Are the various bleeders installed correctly so that they are actually bleeding all of the air out? Generally, yes, with a frame mounted / under floor master you will need residual pressure valves, to keep the fluid from flowing downhill from the wheel caliper / wheel cylinder and back in to the master. But that’s probably not your only problem here. Your master cylinder may or may not have built in residual valves, so you may or may not need to add external ones.
Despite needing a bunch more information than what you listed as others have asked, I'd like to ask if the rear drums are new along with all the brake hardware and shoes. If so, have you had the drums trued up? Have you made sure the shoes are arced to the same radius as the drums? This makes a major difference in pedal feel. I'd like to know much more about your system to diagnose it properly.
David, he already answered several of your questions. I understand that more information would be helpful but so is reading all the posts in the thread before commenting. I think the poor dood is being overwhelmed. Trucker; The master cylinder does most likely have a check valve or two in it. The only way to tell for sure is to disconnect the lines from the master cylinder and look inside the ports. My experience is that it is better to remove the valves from the master cylinder and install individual residual valves for front and rear. 2# for discs and 10# for drum. The problem you are having could easily be a lack of valves in the MC or a valve being defective. Essentially a residual valve hold the brake fluid in the wheel cylinder instead of all being pushed back to the MC, in simple terms it “takes up the slack” in the hydraulic lines. There has to be a lot of air in the system for the pedal to go all the way to the floor. Hope this helps, Don
Yeah, no. The sum total of the original poster’s information is: As you say, reading the messages in the thread is helpful. He has provided pretty much none of what I asked for. He could be overwhelmed. My questions are intended to reduce that by asking for specific details that he either should have, or can relatively easily acquire without a lot of understanding needed. The M/C may or may not have residual valves. Being a frame mounted design, I’d expect that it would, maybe, unless it doesn’t. But without knowing what it is, we’re all just guessing. If he bought a kit, finding out what kit could lead us to knowing whether it should have residual valves in the M/C or not. If it’s not a kit, he’s piecing things together, and may need more or different pieces. It’s “GM” parts on front and back. Could be GM low drag metric calipers that need the extra take up push from the M/C. Could be a drum/drum design M/C pushing in to disk calipers. Could be the wrong size rear cylinders, not matched to the M/C. Could be that he has the front and rear circuits reversed at the M/C. Since it’s some kind of frame mounted design, the pedal ratio is probably fixed and correct, but still may be worth knowing. Could be a lot of things. We have little information and no pictures. I won’t recommend anything in particular without knowing what we’re starting with.
I have a similar setup in my heap, except non boosted, four discs, using 2# residual valves. Factory pedal setup using a two circuit master with 1" bore. It works. I feel the valves help with getting a firm pedal when the brakes are first applied, no matter where the master is located. I use them in every brake setup I've cobbled together. I'd definitely use them on your setup, @old trucke. Good luck!
I don't have the answer on the residual valve but I think its important to bench bleed the M/C using hoses/lines back into to the reservoirs, before installing the M/C into the car. Otherwise it can be very difficult getting all of the air out.
I agree about providing more info, however the pedal going to the floor has no "feel" Arcing the shoes won't cure that. Sounds like a fluid/air problem.
M/C mounted below the wheel cylinders should have residual valves, "2 on the disc and #4 on the drums. At least this is how I build them. But I agree with @squirrel that is not the source of your pedal to the metal issue. Brakes are not a one size fits all product. The bore of the wheels cylinders must be taken into account before choosing the bore size of the M/C. 6:1 pedal ratio is preferable, if you don't know what you have I'd start there. Brakes must be adjusted properly prior to bleeding. The OP needs to start at square one IMO.
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