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Hi all, I've got a puzzling issue going on in a 1976 mb400 RV. It's a 360 automatic that had been off the road for about 15 years. The previous owner had seasonally run the engine off a Jerry can and maintained fluids. The engine was rebuilt about 15k miles ago, which equates to about 20 years or so. She seems to run fine, good oil pressure sounds gd to the ear. Cooling system works and it idles fine. It was parked because of sloppy steering even after new steering gear was put in.
FIrst thing I wanted to address was the brakes. I bled the fronts, and the calipers worked well enough to yard drive. I had no fluid going to the rear, but no visible leaks. I reckon I'll have to replace collapsed hoses and will install new shoes and springs; no big deal
Here's the weird part:
After I initially got the calipers working I let the ole girl sit for about a month. When I went to move it around the yard I applied the brake to shift and white smoke began billowing out from the area of my passenger exhaust manifold! It actually has headers on it (don't ask why, wasn't my doing). These leaked a lot but I tightened the bolts and that had taken care of the majority of my exhaust leak.
So, since she only smokes when brake is applied that was where I started. The master cylinder lost fluid from both reservoirs. Now, as far as I can tell there is only one vacuum connection from the brake booster to the intake manifold which provides the power for the power brakes. Figuring a seal in the MC must have went, and also the valve at the booster, I disconnected the vacuum line and plugged the connection. My theory was brake fluid was being sucked into the engine from the vacuum booster but the hose seemed dry! It also still smokes when I apply the brake, despite the booster being disconnected!
Any thoughts? Only fluid I seem to be losing is brake. Maybe there's another line that connects the booster to the engine that I'm missing...or maybe something else entirely!
Thanks
It'll smoke for maybe 10 seconds or so. After it burns off it runs fine with no smoke. I'm not sure how brake fluid would be leaking onto the exhaust; there isn't any line that runs in the area, but thanks for the reply. Somehow fluid is getting into the exhaust and leaking through that manifold, but it seems to originate inside the engine to me. I will double check, maybe it's spraying from the front passenger brake line onto the manifold. That'd be an easier fix than a master cylinder and or booster!
Thanks for the replies so far!
I would pull the master cylinder and see if it is wet under the booster, you probably need to replace it anyway.
but first pull the vacuum modulator line on the transmission and see if there is transmission fluid in the line, if so the modulator is bad,
Hi all, finally got around to working on the 76 dodge. I pulled the master cylinder and replaced it with a remanufactured unit. There did seem to be fluid leaking past the old MC; looked to be a brown rusty sludge behind the piston where it mates to the booster.
I broke off the front line when removing the MC but connected the rear line with the original fitting. I tested the brakes with fluid in the rear lines without bleeding them. I didn't have any white smoke, and wasn't losing fluid. Problem solved, so I thought...
After flaring the front line and connecting to the MC I attempted to bleed the brakes. I replaced the driver's side caliper and put a new bleeder on the passenger side.
After a few pumps the white smoke was back! Seems to only occur when the pedal is depressed hard, whereas before a light touch would smoke.
This time I was underneath the vehicle watching for leaks in the line...I saw none. The front reservoir on my MC loses fluid, but not the rear reservoir.
I'll pull the new MC to look for signs of fluid loss behind that, perhaps a bad part out of the box? I'm also still suspecting the check valve for the booster vacuum line to the intake.
Any thoughts are appreciated, thanks
The fluid is stuck in the booster.... All the fluid that leaked out of the OEM Master has leaked into the booster.... It will some until it is all gone and then at some point the booster diaphragm will rote out and leak vacuum and the booster will need to be replaced at that point.
Thanks for the reply. While I agree that fluid may be left in the booster from the original MC, I still have fluid loss from the new front reservoir, but not the rear. My lines are solid and no fluid around the calipers, hoses, or MC fittings. I guess I'll bite the bullet and replace the booster but I suspect my new MC is bad as well because, to my understanding, fluid would have to leak past a piston in the MC to get into the booster in the first place.
How could fluid get behind the front and/or rear piston assembly unless there is a defect in the MC itself? Master cylinder only loses fluid from the front reservoir, rear is full Diagram of brake line and intake manifold connection. Specifications of 1976 brake systems. Applicable column is on the far rightVehicle is 1976 mb400 dually 360 auto
AS long as you dont have a leak anywhere else within the system, and the master cylinder is not leaking out the back into the booster. Or the floor of the car?.. The master Cylinder is just by passing from the front to the rear reservoir. So your assumptions would be correct on the faulty master cylinder....
05-26-2022, 07:02 AM
