47-55.1 Electric hydraulic power brakes problem

[TX3100Guy]

Previously, I installed a power brake setup in my 1953 truck. The system consists of a frame mounted master cylinder, along with a small electric hydraulic pump and reservoir. In the case of power failure, the reservoir is supposed to hold pressure to allow multiple stops before reverting back to normal non-powered brakes. I haven't had any issues regarding brake failure, just the opposite.

Since installing the system, I've only had modest use of the truck due to sorting out other various issues that I've posted on this forum (engine rebuilds, oil leaking, over heating, etc.). I have sorted them out one by one (with help from many of you) and wanted to begin driving the truck more.

During the overheating stage, I noted in the back of my mind, that after the truck ran a while it was taking increasingly more RPM's to get the truck moving from a stop, but I was mentally busy chasing the overheating issue (largely a timing and mixture issue) and didn't pay much attention to this situation.

Yesterday, with what I though was all issues under control, I took the truck on a modest 20 mile circuit and barely made it home. About ten miles into the trip, I noted again the RPM's necessary to get moving from a traffic light and then realized that I was unable to get the truck moving fast enough to get into third gear. I limped home in second gear, accelerator to the floor at about 15-20 mph with the truck at about 1500 - 2000 RPM.

When I pulled into the shop, I pulled in just enough to close the door, shut the engine off, which was beginning to overheat given the driving circumstances. I then tried to push the truck a few feet further to align it with my lift and found all four wheels locked up. I even put a floor jack under the rear axle and could not turn the wheels at all (yes I was in neutral). The jack under the front axle yielded two wheels that would just barely turn with much effort. It appeared that despite no brake pedal being applied, the four wheels were locked up tight.

About 30 minutes later after the truck had cooled and apparently the hydraulic pump had released some of its pressure, the wheels turned freely. At first, my thought was that the pump was not stopping when it reached the pressure required to switch it off, but then I realized that with the pump on or off, its pressure line to the master cylinder was always high, the master cylinder must be allowing that pressure line to seep through the seals of the piston to engage the front and rear brake calipers.

I have an email into the company that supplied the system www.abspowerbrakes.com but have not heard back from them yet. Today, I removed the relay to the pump so that the truck only has manual brakes, since the pump will not operate without the relay. Attached is a photo of the system diagram, any thoughts or advice are always welcome.

[leegreen]

So that is a master cylinder with built-in hydraulic power servo? I have no experience but my thoughts are that it is either a pedal adjustment issue, where the pedal is putting some pressure on master at all times or there is something wrong with the power boost servo and it is always applying braking force.

It sounds like it has gotten worse which would seem to indicate the servo failing. If you disconnected everything but the 2 lines to the pump and ran the pump I wonder if you'd find fluid coming out the two brake line ports. It would be a difficult and messy test, see what the supplier has to say first.

Since your build is all new stuff I'm assuming you don't have water in the brake fluid boiling to steam and locking brakes up, or sticking caliper slides, or overly tight drum brake adjustments, or collapsed flexible brake lines, those could all cause locking up brakes.

[TX3100Guy]

Originally Posted by leegreen

So that is a master cylinder with built-in hydraulic power servo? I have no experience but my thoughts are that it is either a pedal adjustment issue, where the pedal is putting some pressure on master at all times or there is something wrong with the power boost servo and it is always applying braking force.

It sounds like it has gotten worse which would seem to indicate the servo failing. If you disconnected everything but the 2 lines to the pump and ran the pump I wonder if you'd find fluid coming out the two brake line ports. It would be a difficult and messy test, see what the supplier has to say first.

Since your build is all new stuff I'm assuming you don't have water in the brake fluid boiling to steam and locking brakes up, or sticking caliper slides, or overly tight drum brake adjustments, or collapsed flexible brake lines, those could all cause locking up brakes.

Lee, thanks for responding so quickly. Let me answer a few of the questions you raise. The master cylinder (see photo above) has two low pressure fluid supply lines, one supplying brake fluid from the firewall mounted remote reservoir and one line returning fluid back to the reservoir, as needed. The MC also has three threaded brake line ports, one for the front brake circuit, one for the rear brake circuit, and a high pressure line from the electric hydraulic pump.

Today, I also revisited the shaft from the MC where it mounts to the factory brake pedal bracket. It has a threaded heim joint and jam nut to adjust its length. I removed the 9/16" bolt holding that joint to the bracket and noted that it was slightly tight getting the bolt in and out, but adjusting the threaded heim joint one or two threads made it easy to slip the bolt in and out. Total relaxed distance about 1/16 of an inch.

The MC doesn't have a servo, the electric pump must have that. The pump has wiring to it, a brake fluid supply line from the remote reservoir, and the hard/high pressure line from the pump to the third threaded port on the MC.

I too assume no water in the brake system since every inch of it is new, including the calipers on all four wheels. The rubber lines at each caliper are also new, as are a couple of the flexible stainless steel lines that navigate some tricky turns.

By removing the relay from the pump, I'm trying to accomplish what you are suggesting by reverting to manual brakes versus ones where the hydraulic pump continues to feed high pressure fluid to the MC. My plan is to take a short drive in my neighborhood with the manual brakes to see if the issue repeats itself. My suspicion at this point is an internally leaking MC where pressurized brake fluid is leaking past the bore seals on the brake piston allowing for continued pressure on the brake lines/calipers.

[leegreen]

I guess another test would be to jack all 4 wheels up and run that pump for a while, see if the brakes start dragging

[TX3100Guy]

Originally Posted by leegreen

I guess another test would be to jack all 4 wheels up and run that pump for a while, see if the brakes start dragging

Just had lunch with an old friend who worked on cars in his younger days, but was an oil field engineer with hydraulic systems. He agreed with the thought that the master cylinder is likely the culprit and suggested doing the same thing that you just did, so when I came home I gave it a try.

The test was somewhat inclusive. I lifted the wheels off the ground on the lift and was able to rotate all four of them easily. The pump will run when the key is switched to accessory, after turning the key on (not running the engine) I heard the pump run for a few moments re-building pressure, After 15 minutes, the wheels still turned easily.

I then decided to make the test more like real world, I got in and pumped the brakes multiple times. After every few pumps, the hydraulic pump would run for a few seconds to re-build pressure. I would check the wheels after every few minutes and felt the rear wheels begin to get significantly tighter to turn. After thirty minutes, one rear wheel would not turn, and the other just barely, but the front wheels never fully stopped spinning, but they did get somewhat tighter. Other than the one rear wheel, I did not get a full lock up, but something isn't right.

[dsraven]

I would first check all the brake pedal linkages and the pedal stop to ensure there is nothing worn or loose that would allow a differences in where the pedal is allowed to stop when you release the brakes. if the pedal doesn't have a hard stop point, so that point is allowed to be slightly different from time to time, then that will have the effect of possibly keeping some pressure on the master cylinder which doesn't allow the master cylinder piston to return all the way back. which means the fluid in the lines can heat up and expand from heat transferred from the action of braking and the expanded fluid is trapped in the system. so the brakes can get tighter as the brake fluid warms up and expands in the lines, which basically applies the brakes, slightly at first but then more and more as the brakes get hotter and hotter.
this is a common problem with mismatched booster and master cylinders or when a new master cylinder is installed but the booster pushrod, that pushes on the master cylinder piston, is not set up to have the correct tolerance/clearance between the pushrod and the piston. basically the master cylinder piston is pushed past the ports that allow fluid to return to the reservoir when the master and the booster are assembled
kind of a quick test that i use is to put on safety glasses, remove the fluid reservoir cap, and have a buddy apply the brakes. if the master cyl is set up with some freeplay then there will be a small disturbance in the fluid, like a small bubble or fountain of fluid right when the pedal is first pushed. then nothing after that. this is because the piston is behind the ports at first, so moving the piston forward pushed some fluid back up into the res. when the piston passes the ports the disturbance stops and brake fluid is moved down the brake lines.
just a thought that may help you figure it out

[dsraven]

this link takes you to a page with a diagram of the master cylinder ports

otherwise, unplug the hydraulic pump and see if you still get the same thing happening. this eliminates a hydraulic pump issue

[leegreen]

Does this still happen if the cap on the reservoir is left loose? I'm wondering if the pump heats the fluid in the common reservoir and the air in the reservoir expands, pressurizing the brakes. brake reservoirs do not usually have much of a vent

otherwise the master spool must be moving far enough to block the inlet ports and it is prevented from coming back, either mechanically or hydraulically,

[nvrdone]

OK, I don't have any experience with this type of system, but I had the same type of problem when I did a power brake conversion on my '49. You mentioned that your m/c etc is all mounted under the floor. Are you using a stock pedal or an after market? In my case I have a stock pedal. My problem was two fold. First, the pedal return spring wasnt strong enough. Second, when I would turn a corner the cab would shift, causing the pedal to bind where it goes thru the floor. Took relpacing the cab mounts and some serious adjustments to get it right.
Just throwing this out as another idea. Good luck.

[TX3100Guy]

Hopefully my responses answers all the suggestions offered.

The brake pedal returns to the upright position smartly. I'm using the stock pedal and spring mount with the master cylinder push rod attached to the arm via a heim joint. With or without the push rod attached, the pedal snaps back and even thumps as the rubber block on the bracket hits the firewall.

The remote fluid reservoir on the firewall has three low pressure lines attached to it. One feeds the electric hydraulic pump, one feeds the master cylinder and one is the return line from the master cylinder to the reservoir. When the pedal is pressed, the fluid pressure in the pump reservoir (attached to the pump) releases pressure via a hard/high pressure line to the MC. Once the pedal is released this extra fluid is returned to the remote reservoir. Once the pump reservoirs pressure drops, its sensor turns the pump back on to re-build pressure.

I plan on driving the truck around the neighborhood (so I can limp home if necessary) with the pump relay disconnected so that I just have manual braking with no pressure assist from the elecrtic pump. More to come and hopefully I hear from the manufacturer tomorrow.

[nvrdone]

If nothing else, buy a boat anchor & throw that out the window. LOL!!

[dsraven]

If it still does the same thing, then the next quick test is to try installing a shim between the master cyl mount flange and the booster. Like a couple washers on the master mounting bolts/studs. Try it again. If it's fine then that failed test and the adding a shim test points to a booster output pushrod being slightly too long (that pushes directly on the master cyl piston). The shim test is just spacing the master away from the booster to create clearance and not waste time setting up tools to do the actual booster pushrod adjustment procedure.

[nvrdone]

Raven's got a great idea. Another quick test is t push the brake pedal down and see how far it travels before you feel resistance.

[dsraven]

again, going back to linkages and pivots and those things.
the general workings are firmly mounted on the truck frame. no room for things to move because they are bolted to the truck frame
the cab, where the pedal comes through the floor, is not firmly bolted to the truck frame but is cushioned and so has the ability to move around some
your pedal stop is relying on the firewall of the truck cab as it's reference
possibly the problem is coming from the pedal stop reference point
if the cab settles when you are sitting in there then your pedal stop also is being moved
just something to think about
try removing your floor seals around the pedal shaft, mark the pedal shaft with something that can give a fine line, drive the truck till it does the brake problem, check to see if the refernce line has changed
if it has changed even the slightest then that could be your problem, the cab settles, or moves, when you use the truck
more solid mounts for the cab may help, or a better way to set up the pedal stop, or a little more room for cab movement built into the pedal geometry
just a thought I had sitting here thinking about it

[nvrdone]

Raven: You and I are thinking along the same lines. I finally solved the problem with 2 changes. First I took a step drill and opened up the hole in the floor board. Also it was slightly out of round. Second I replaced the front mounts with poly mounts. Then replace the bushings in the shackles also with poly. So far that seems to have helped.

[leegreen]

Since it can be recreated on the hoist, get the brakes to lock up, then try three things:

pull up on the pedal, just in case and since it is easy to do

open the cap on the reservoir, if that releases brakes I'd probably run separate reservoirs so the master side was not effected by pressure from pump side rather than increase venting and risk water contamination.

back off the nuts holding master to bracket enough to so you can move it back and forth a bit with some slack at pushrod, if that releases it there is an issue in pedal/linkage/pushrod to sort

if the brakes are still locked with reservoir open and master loose on it's mounts then the issue is internal to the master. To confirm it crack the front and rear lines open, if brakes release it is the master holding pressure in the lines

[dsraven]

in your brake system do you have proportioning valve or combination valve, residual valves, a master cylinder with built in residual valves?

Leegreen makes a good point in that pulling up on the pedal, when the brakes are locked, may release the brakes if you have a linkage problem. it's a simple and quick test too. then cracking a brake line at the master is also a good tell to see if it's the master holding things up. and, a seperate reservoir for the hydraulic pump seems like it should have been engineered that way. does the fluid in the reservoir get warm from running the hydraulic pump? if so, the res cap may not have a good enough vent for the purpose

[TX3100Guy]

Did all the testing suggested and the brakes did not release. There are no valves between the master cylinder and the brake circuits. The MC pushrod is loose when I pull the bolt from its heim joint and the pedal linkage.

After talking to tech support at the manufacturer, they are stumped too and think it likely is a bad master cylinder. They have asked me to end them both the electric hydraulic pump and the master cylinder. They will inspect, test, and repair as necessary. While this is a messy job, I need to sort out this issue, so tomorrow is removal day.

[dsraven]

have you tried the shim between the master cylinder and the booster? that pushrod exitting the booster could still be holding the master cyl piston from returing fully.
it could be a faulty booster as well. if the manufacturer ia willing to have a look at it, or simply exchange the whole unit, then I would suggest going that route.

[TX3100Guy]

Originally Posted by dsraven

have you tried the shim between the master cylinder and the booster? that pushrod exitting the booster could still be holding the master cyl piston from returing fully.
it could be a faulty booster as well. if the manufacturer ia willing to have a look at it, or simply exchange the whole unit, then I would suggest going that route.

There is no booster between the master cylinder and the frame mount. The system gets its boosted hydraulic pressure from an electric pump mounted on the other side of the frame. There is a hard/high pressure brake line between the two.

Hopefully, the manufacturer will test and repair/replace both the pump and master cylinder that I removed and sent to them yesterday.

[dsraven]

yeah, fingers crossed. hopefully you can get this thing done and on the road so you can enjoy it without the dread of a breakdown hanging over your head.
if that system seems to be prone to problems you can always swap in a normal hydroboost on the frame with stock pedal set up. it's been done by members on the site. you would need a power steering pump though, or an electrically driven pwoer steering pump I suppose.
lets just keep our fingers crossed that that the system you have will get fixed and work fine.