I suspect a BAD "proportioning block". I installed a DUAL MASTER cylinder with 10PSI RESIDUAL VALVES and a PROPORTIONING BLOCK. I can NOT get the brake pedal to stay up. When I clamp the LH front brake hose, the pedal is PERFECT. Both front wheel cylinders are new and the entire system has been bled. Has anyone had similar problems ? My next step is to "by pass" the proportioning valve all together. Below is a drawing of the installation:
the fronts should not run through the PROPORTIONING valve at least thats the way mine is it only controls the rears on my car
What master cylinder are you using? I would suggest taking out both residual valves AND the block. THEN find out if you need them. Most of the master cylinders I have used on 4 wheel drum with the master cylinder under the floor, did not need a residual valve or any porportional valve.
Sounds like the problem is in left front if the pedal is good when it is blocked off. Is the master cyl on the firewall or under the floor?
I see a couple of things going on here. On a drum drum brake system a proprotioning valve is not needed. If it was though, it would be plumbed to the rear, not on the front. If you have clamped of the LF and have a good pedal, the problem would be in the LF. Probably out of adjustment, or a hydraulic leak. Just my 2 cents. Lets us know what you find. Bob
Discs use a 2 pound RV, drums use a 10 pound RV. The main purpose of an RV for the drums is to keep the wheel cylinder caps sealed, so as not to suck air into the system on releasing the pedal. The location of the MC does not determine whether or not an RV is needed. It is needed. FWIW I have a drum-drum system on my rod (F-100 front, S-10 rear), a single pot MC, and one 10 pound RV.
ANSWERS TO SOME OF THE ABOVE QUESTIONS: MC is dual reservoir purchased from SPEEDWAY-(installed under floor) PROPORTIONING BLOCK, Speedway- #91031358- (meant to be used with ALL 4 drums & to connect to FRONT & BACK wheel cylinders) per drawing above Residual valves SHOULD be used when MC is under floor. I rather ERR on the side of CAUTION! The SPEEDWAY tech confirmed that the LF wheel is isolated and IS where the problem lies. I had the problem BEFORE I replaced BOTH front wheel cylinders with NEW ones. There are NO leaks to be found either. HENCE MY DILEMMA!!!!!!!!!
That "proportioning block" should only be used with disc/drum or disc/disc, not drum/drum. The one exception could be early inefficient front drum brakes with modern rear drums. I would suggest you simply run the front brake line from the master cylinder to a tee for the fronts, and the same for the rears. The residuals are correct, either 10 lb. external or OE internal in the master cylinder, and needed for all vintage drum brakes mid-'70s and older, regardless of master cylinder location. More detail on just what kind of vehicle and brakes you have may help. Are the brakes all adjusted properly?
Could it be that you have large volume front wheel cylinders, and the master cyl. can't provide enough fluid to the front. That could be why it works when you clamp off the left front, you are reducing the required volume by half and the m/c is then able to function. I also agree you don't need a prop. valve with drum/drum. You only need a residual valve if the m/c doesn't have an internal one.
MY ride is my "avatar"........1930 Sport Coupe Model A with a 283CI and 4L60 tranny- Before I purchased the PROPORTIONING BLOCK I asked if it could be used with DRUM/DRUM and was given a POSITIVE. The FRONT brakes are complete 1969 Buick Riviera assembly and the REAR END is a 1991 S10 Chevy truck. (quite extreme front brakes for a Model A) I can INCREASE or DECREASE the REAR brakes as needed with the adjustment on the PB. I currently have it adjusted to MAX DECREASE for the rear............not really concerned whether I have to adjust the back brakes or not. What I really like about the PROPORTIONING BLOCK is the brake switch. I think someone might have something here........REF NOT enough volume to the front cylinders. Although, the front reservoir is twice the size of the rear brakes...... Something I will have to investigate further.........if it still does this once I spend a few more hours BLEEDING the darn thing! The PB gives the OPTION of running separate lines from the PB to each FRONT wheel or one can run both wheels off of one port out of the PB. I chose separate ports to give each wheel its own line from the PB and perhaps more juice as well. (per the drawing) The TECH I spoke to this morning was really "hung up" on the fact the PROBLEM is isolated to the LH front wheel. Another words, everything behind the cut off point (front brake line) is just fine since I get a FULL pedal. Another thing to consider is that the LH front wheel cylinder is quite a bit closer to the MC than the RH and the line is much shorter as well. Sure hope it all ends up being too much air still in the lines. Doing the "gravity" thing at the moment that so many brag about until I get my "mechanic's" little helper to free up some of her precious time to pump on the brake pedal. SUCH A DILEMMA. Thanks a bunch for all the input............. LG, Mike
Here's a video from Muscle Car describing under floor mounted masters, just move near the end of the show. http://www.powerblocktv.com/episodes/MC2013-02/oldsmobile-restomod-returns I had a lot of trouble bleeding the dual master (no proportioning valve, stock brakes), ended up being a bad master. New one fixed it. Sounds like your getting fluid draining back to the master although the residual valve is supposed to fix that. Have you tried a pressurized bleeder? I know of someone that made one out of a bug sprayer.
¿Brake adjustment? I adjusted the brakes right after installing new WHEEL CYLINDERS in both front wheels and BEFORE starting the bleeding process. Tightened them until I could NOT move the drum and then backed them off turning the drums to where I felt adequate. The DRUMS have been turned and new shoes installed. I don't think the adjustment is the problem. I will admit that before I reinstalled the drum the second time around on the LH wheel, I pushed on the pedal slowly to see how far the shoes would move outward. Sadly to report that they did NOT move as much as I expected.........the front one a little more than the rear shoe which was practically not noticeable at all. It is definitely for lack of VOLUME either do to air in the line or NOT ENOUGH volume like PETE EASTWOOD suggested. I will DEFINITELY post what the problem is once I find out-
I think that CUTTING OFF the fluid to the FRONT BRAKES would have determined a BAD Master Cylinder if the pedal dropped to the floor. This is MY DILEMMA...........I get PERFECT brake pedal with CUTTING OFF the LH FRONT wheel. Another words, isolating the problem to the LH Front WHEEL CYLINDER.
Drum/drum masters have the same size chamber for front and rear, if the master is for disc/ drum you may not be getting enough fluid.
When you isolate the LF, you may just be getting enough volume to operate the RF, but this would show by reversing the process and isolating the RF, which I presume the OP has done. Seems to me the only answer, having read every post, is a bad LF cylinder.... it happens.
Did you replace the front hoses?? Also did you try blocking off the right front?? Dont remember seeing anything about that. My experience in trying the gravity bleed route with resid.valves, does not work. No flow throught he valve. Did you compare LF and RF for travel??
did you replace the front hoses?? YES Also did you try blocking off the right front?? YES- PEDAL GOES TO THE FLOOR I DID NOT COMPARE THE LF TO RF TRAVEL. BRAKES WERE ADJUSTED PRIOR TO BLEEDING ............... This is a NEW build so pretty much everything is new, although a LOW BUDGET build.
To me your drawing doesn't look right. A combination valve is a metering valve, failure light switch, and a proportioner. It does not care if it is drum or disc brakes. A combination valve works when you put pressure to the master cyl, it lets a small amount of pressure to the rears, to take up the slack in the shoe to drums, and then sends all the pressure to the fronts, then lets the remainder pressure to the rear wheels. This so that you don't lock up the rear brakes before the fronts. Because your stopping force is with the front brakes, not the rear. A metering valve just meters the pressure to, light rear, then full front, then full rear pressure. Very quickly! Here are two pictures of the two different valves.
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