Hydro Vac brakes- 1980 International

coonie minnie · Nov 10, 2017

So my grain truck has been having issues- I think it is afraid of harvest!

It has the hydro-vac brake system with a vaccuum pump on the engine. Here is the issue- if you attempt to stop, there will be plenty of "pedal", but after a few seconds you will loose all power assist, and you can stomp on it with both feet and not stop!

the odd part is, it will work just fine the next time (about half the time) but then again you could be with plenty of pedal and NO stopping power.

My mechanic has replaced the hydro vac booster (it was new last year, still under warranty) and has spent hours and hours scouring the thing for something wrong, but has yet to find it.

He called last night ready to give up. Any ideas anyone???

I should add that when the problem occurs, the vaccum gauge still reads 20 plus lbs of vaccuum.

Bob · Nov 10, 2017

The brakes need to be set up at each and every wheel.

If there's too much travel in all the brake wheel cylinders together and the hydrovac reaches the end of it's stroke before the brakes are tightly applied, there can be no more power assist and the pedal feels "hard".

Hobo,NC · Nov 10, 2017

I will add make sure the booster vent is clear.

Hobo,NC · Nov 10, 2017

And read the vacuum as close as you can at the booster what the dash gauge reads and what is at the booster may be different.

Proper brake adjustment is the #1 suspect tho....

A red shop rag used to plug off the vent will throw you a curve ball that's hard to hit BTDT....

WGM · Nov 10, 2017

I?d be replacing brake shoes. Had the same issues a couple years ago. Brake shoes were worn out. Replacing them solved the problem.

WGM · Nov 10, 2017

Hydraulic brake systems don?t have a dash vacuum
gauge.

37chief · Nov 10, 2017

Not the same problem you have, but I experienced braking troubles with my chevy flat bed. The hydro vac on my truck is operated by motor vacuum. The Hydro Vac seals were bad, sucking brake fluid into the engine draining the master cylinder. Then very poor, or no brakes. It took a while to figure where the fluid was going, since I cold see no leaks. A rebuilt Hydro Vac fixed my problem. Stan

Swede in IOWA · Nov 10, 2017

With age the vacuum hose from the pump to the booster can deteriorate on the inside and can collapse stopping the vacuum boost. Swede

Welding man · Nov 10, 2017

You never owned a Ford F-700 then. I have owned 2 that had dash vacuum gauges.Also had one that had a low vacuum idiot light.

coonie minnie · Nov 10, 2017

this one does.

coonie minnie · Nov 10, 2017

Thank you all for the replies. We'll see where we get.

Ken Combs · Nov 10, 2017

No gauge, but my Mitsubishi(sort of 1T size) has a low vacuum warning light. Diesel with vacuum pump on the back of the alternator. Reminds me of the '50s cars with power steering pumps on the back side of the generator.

pete 23 · Nov 10, 2017

I would install a reserve tank if it does not have one. Even good braking systems can get you in trouble if you pump the brakes a couple times, even inadvertently, when you least expect it. Would do same thing if brakes were needing adjustment and the limited volume of vacuum chamber in hydrodvac is not sufficient. . . I don't know where your vacuum gauge is hooked, but apparently it is not where the hydrovac unit is directly supplied. With a dead engine, you should be able to brake several times with boost if you have a reserve tank. I like to pump the brakes on dead engine until hard and then start engine and the pedal should soften as a proper brake booster is working the way it should.

RustyR · Nov 10, 2017

the hydro vac system uses a check valve somewhere in the vacuum hose. these used to go bad and cause screwball problems. hose could be collapsing also

dairyman · Nov 10, 2017

Another thought for you brother has a dodge d500 with the hydrovac brakes we actually got a junk reman booster that acted the way you describe. If I remember right we found out that the diaphragm in the booster was garbage. I think there are 2 lines on the booster one is a vacuum line the other is ambient air pressure, what we did to find the issue was disconnect the ambient and start the truck and check it for vacuum. If there is vacuum there you need a booster though you can get by plugging the ambient line but if you do that the power assist on the brakes does not work just manual brakes. Hope this helps.
Dave

marloweg · Nov 10, 2017

set the brakes up each wheel

dairyman · Nov 10, 2017

Another thought for you brother has a dodge d500 with the hydrovac brakes we actually got a junk reman booster that acted the way you describe. If I remember right we found out that the diaphragm in the booster was garbage. I think there are 2 lines on the booster one is a vacuum line the other is ambient air pressure, what we did to find the issue was disconnect the ambient and start the truck and check it for vacuum. If there is vacuum there you need a booster though you can get by plugging the ambient line but if you do that the power assist on the brakes does not work just manual brakes. Hope this helps.
Dave

flying belgian · Nov 10, 2017

I'm going to go with swede on this one. The plys on hose can separate and collapse. Looks good on the outside but is collapsed on inside.

VicS · Nov 10, 2017

After what you have said. Adjust your brakes! Your booster is reaching the end of its stroke before shoes hit the drums. Tighten the rears till you can't turn them by hand. Then just back off 1 click.

sander · Nov 10, 2017

(reply to post at 03:58:19 11/11/17)

rought IH truck with hydro vac brakes. Would not stop. Full pedal, rock solid with engine rpms up, kinda worked at idle. When new unit was installed they'd hooked up the vacumn lines wrong.

K Effective · Nov 12, 2017

On a similar note, I have a 73 Loadstar with the Hydro-boost brakes as well, Can I vacuum bleed the lines to the wheel cylinders on a system like that- (no reservoir at the booster)? When I picked it up we bled all six brake cylinders, but had no brakes still. That's when I found out we were supposed to bleed the hydro first. I did bleed it now, but still have no brakes. Feels like the left rear is hanging up when I try to drive it...

Hydro Vac brakes- 1980 International

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