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Bleeding brakes

Bleeding Brakes

Gents - All this talk of binding brakes recently got me interested to complete my brake system this weekend. My frame-off resto of '53 M FHC is nearing completion and all I had left to do was to bleed the brakes, I thought !!!! Here's what happened: The entire brake system was rebuilt, master cylinder (tandem), new tilt valves, all wheel cylinders, new hoses, lines, etc. while the chassis was being restored. I filled the resevoir with DOT 4 fluid, open each wheel's bleeder until fluid drips at each corner. I went through the bleeding procedure and all seems to be right with the world. After the last corner (left front) was bled, the pedal still goes all the way to the floor, with almost no more effort than while bleeding, and while holding the pedal to the floor (maximum braking effort), the rear drums barely registered any drag and the front drums spun freely. The pedal does not pump up, nor is it spongy. I'm rather discouraged by this, as I have rebuilt dozens of brake systems (my father taught me to do this first - a car with other mechanical problems is a nuisance, but one that won't stop is a casket). Anybody got any ideas? I guess the master cylinder is the problem, but with a new rebuild kit in it, honed lightly, properly reassembled, I can't imagine what could be so seriously wrong in there. Thanks for the help. - Scott Selbach, S680701

Scott - Maybe the brake shoes are not preadjusted close enough to the brake drums. On the 140 the brakes are self adjusting and you have to keep pumpimg them to rachet them into a position very close to the drums. I think that position is in the order of thousandths like .060 inches and not like a quarter inch or so. Once one brake pad contacts the drum, that wheel cylinder can't move any further and the pumping action starts to work on the remaining cylinders. They in turn adjust themselves. In the end, brake pedal action of say an inch or so is only moving the brake cyliners about .060 inches until they contact the drums. Try adjusting each brakes mechanical adjustments, spinning the wheel by hand until they lock, and then back off slighly so there is a little drag ie. the pad is very close to the drum. Then see how the pedal/master cylinder feel. - Regards; Andy Leavitt

My 53 FHC S ( or M) (S681221) has exactly the same problem. Everything rebuild, everything bleeded. No pressure! It can't be the year, can it? - Duco Avis

Scott: 120 tandem-cylinder brake bleeding has raised my blood pressure on more occasions than I wish to recall. There are a lot of people on the list very qualified to address this but here's what I think is part of the problem. First-off, observe the fluid level in the reservoir and make certain it rises only slightly when the pedal is depressed. A large rise can signal a bad tilt-valve which is allowing the fluid to return to the reservoir instead of going to the brakes. 1. There are several high points in the routing of the brake lines where I believe air becomes trapped. 2. The trapped air will only be expelled by a high fluid flow-rate, not by the flow-rates generated during a normal, genteel bleeding process. How do you get a high flow rate? It can be partly accomplished by having the "pedal-pushing assistant" pump-up the pedal to the highest level possible before the "bleed screw operator" gives the screw a good 1/2-3/4 turn open. The "pedal-pusher" must force the pedal down rapidly, and hold it down, until the bleed screw is closed. The process is repeated until one gets a firm pedal without pumping. If available, a professional pressure-bleeding device, which attaches a pressurized supply of brake fluid directly to the reservoir, can also help. Alternatively, I made a special reservoir cap which has a quick-disconnect air fitting on it, to which I attach a low pressure air supply (less than 10 psi). This turns the process into a one-person operation and, except for having to refill the reservoir fairly often, it is convenient to use. (It's very easy to tell when that section of the reservoir is empty.) :-) The latter system has not been the final answer in every instance but it has certainly helped. I have never had any luck using vacuum devices at the bleed nipple. It always appears that air is sucked past the bleed screw threads and again, the flow-rate is low. I hope someone has a better way. - Good luck, Dick Cavicke, 120 OTS & FHC

Dear Sir, I too had trouble pumping up the brakes on our xk120 DHC. It acted like the piston would go forward inside and stay forward, undetected, because the peddle would return. It finally started working when I had someone pump the peddle 4 times and hold down, while I quickly opened and closed the valve before the pressure was completely lost. It really took off after the brake springs came into play to help push the master piston back to it's original position. I may have imagined the non returning piston because I could not see inside, But that is what it acted like and it works now. - Edgar Blake

I have used the small handheld vaccuum pumps with excellent results, but you have to make a collection jar for it to work. I use a glass mayonnaise jar, with two 1/4" O.D. copper tubes soldered to the the lid. Both tubes protrude about 1" out of the top of the lid, allowing you to slip on a rubber hose. Inside the jar one of the tubes descends to within 1/2" of the bottom,the other exits 1\2" below the lid. Have the newly rebuilt system topped up with fluid, and all the bleeders closed. Go to the farthest bleeder first, remove the bleeder screw, and dab some silcone sealant around the screw threads, install the bleeder, finger tight, close and then open 1 turn, attach a rubber hose from the bleeder to the copper tube that is within 1/2" from the bottom. The jar should be primed with the new brake fluid to a level that just covers the copper tube at the bottom. Then you connect the pump to the other tube,pump the hand vacuum pump, bubbles will emerge out of the submerged copper tube, when the bubbles stop, close the bleeder. Keep checking the level of fluid in the reservoir, and repeat the bleed procedure at each wheel. I also follow up with a pressure bleed using an assistant stepping on the brake pedal. I use the same mayonnaise jar, again watching for bubbles. If there are no bubbles go on to the next wheel. - Regards, Wray Schelin

>From personal experience of wrestling the brakes on a MK V to death (or should I say life?) no single thing will get the brakes working. It's down to everything being right at the same time. You need to follow all the good advice in this thread and have the shoes properly adjusted, the pivots smoothly operating, NO AIR in the system, good flexibles ets. Like you I started from a complete strip down (new seals everywhere, new cylinder in one corner, new pipes throughout, new shoes/linings, fluid (silicone)) and you really do have to go through the exercise of pumping the pedal to exercise the shoes etc. When everything as as per spec, it'll all work. just like they did it in the factory. - Dick Clements, '51 Mk V 3.5l

Scott - In addition to all the other good comments, let me suggest another. I know you said you have properly assembled the tandem master cylinder. But I am curious about the lack of resistance at the pedal. By your description, it appears that the master cylinder is not working as a pump to pressurize the system. Could you have nipped a seal as it passed through a port? When the manuals call for use of a blunt instrument to depress the edge of the seal as it passes a port, they mean that. I have ruined more than one seal when I did not heed this simple assembly procedure. Are you certain you installed the seals of each piston to face the correct way? If one or more of the seals have been nipped or installed backwards, the pump won't pump. This past fall I rebuilt the tandem master cylinder on my 120 SE OTS. The brakes had worked perfectly due to a previous complete system rebuild similar to what you described on your FHC. I rebuilt the master cylinder as a result of the seals failing from the use of silicone brake fluid. Anyway, I was anxious to get the brake system working again. With an assistant I pressurized the system similar to what Wray described and bled the wheels from the farthest back to the front. Rear brakes were excellent but front were freely spinning with no drag. I get some pedal as a result of the back working OK. While I was very careful to properly reassemble the master cylinder, I dreaded to take it off and start again. I haven't gotten back to the task but I know what I am first going to do with my problem. Someone on the list recently mentioned the role of the tilt valves. Since only the front are not working, I am going to remove the fluid from the reservoir and line to the front tilt valve, remove this valve and check for a bent pin. I suspect that I may have bent the pin upon assembly. At least this is the easiest thing to approach before I remove the cylinder and dissassemble. Anyway, I might not have pushed the piston in the right position when I tightened down the tilt valve controlling the front wheels. I'm going to cross my fingers and hope a pin is bent. Another thing I am going to do is to make a device to hold and test a rebuilt master cylinder before installing in the car. This will be a bench test device. As you know it is such a pain to remove the cylinder once installed. These brake systems are too straight forward to have little things trip you up after a rework. So, if I can bench test the cylinder before installing, this will be one major component that won't have to be dealt with if something is wrong overall. Good luck. - Bob Oates

update 8/27/98...

During our past discussions of 120 (tandem cyl) brake bleeding methods, I hadn't thoroughly read, practiced nor heard of the specific method... listed in the Service Manual. :-( . Section L, Page L.41, under BLEEDING THE HYDRAULIC SYSTEM (Later XK 120 Cars) states: "As a tandem master cylinder is fitted, the best result will be obtained by bleeding one front and one rear wheel cylinder simultaneously." Has anyone tried or normally used this "duo-bleed" method? Could it possibly be one solution to the difficult problem of getting all the air out of the system? Of course it might be just one more source of spousal irritation when you keep yelling for your wife to.. " YES, KEEP HOLDING the PEDAL... ALL THE WAY DOWN", as you are crawling around closing two bleed screws instead of just one. - Regards, Dick Cavicke

Dick: Boy, that 'wife' comment sure rings a bell. I bought a vacuum bleeder to use until my kids get old enough to push the pedal and to use again when they get old enough to gripe about it! - Regards Bruce Baysinger

When I did the brake job two years ago, I did use the bleeding proceedure in the manual (one front and rear at the same time) and it worked beautifully. (BTW - it is still a mystery to me why a bleed valve on the bottom of the assembly gets all the air out but it does work.) - Bruce Cunningham, Seneca Creek Road, Hinesburg, VT 05461, '53 XK120 OTS

When I bled the brakes, I attached tubes to the bleed nipples and put the end of the tubes in glass jars on the floor. I could then reach in with a stick and push the brake pedal down while watching for the bubbles to stop in the jars. According to the manual, you just work the pedal slowly with the nipples open so you don't need a second person. I used a stick that was cut to a length to prop against the steering wheel to hold the pedal down while the nipples were being tightened. -- Bruce Cunningham, Seneca Creek Road, Hinesburg, VT 05461, '53 XK120 OTS

Bruce, thanks for clarifying the brake bleeding question. Now, do you need two people, to open the bleed screws simultaneously? Love your proverbs! - Phil Maurice

I took an old reservoir cover and managed a valve in the center. Pay attention to the gasket.Then I filled up the Brake reservoir, put a pump on the valve and set the system under pressure(~3bar). So it´s quite easy to do the job alone and it take half the time. Important is during bleeding to check the niveau of the reservoir. - Thomas Wagstyl, Germany 120 OTS

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