Rock hard brakes won't

I have switched to vacuum assist, 1989, I replaced the fluid reservoir seals and flushed in all new fluid, dang my legs are tired, now when.I hit the brakes it takes both feet on the pedal and a lot of force to stop the car! I need ideas on what to look at.

Do you have good vacuum to the power booster - the symptom you describe is what is expected when the engine is not running, so no vacuum present - Tex.

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Yes, I disconnected the hose at the check vale plenty of suction.

That is good - you may have a bad check valve, or may have a bad mounting grommet not sealing the port on the booster housing - worst case you may have a bad diaphragm in the booster - Tex.

I am just passing on what I have found in my experience - do a google search for “how to check brake booster check valve” and you will get some good info (i just did it to see if it would have info to help you) - Tex.

is that red piece a check valve attached to the assist?

David …

Remember the brake booster doesn’t have anything to do with the amount of braking that’s applied to the wheels. That’s determined by the brake pedal pushing on the master cylinder which hydraulically powers the pistons in the brake calipers to push the pads against the rotors. Of course you knew that, basic stuff but sometimes overlooked.

All the vacuum booster does is assist you in pushing down that brake pedal, that’s it.

  1. How much vacuum is your car producing? Generally it requires at least 18in of vacuum at idle.

2 What size and kind of brake booster are you using? For this modification an 8" dual diaphragm booster is generally used.

  1. Do you have an inline check valve. Be carefully because sometimes vacuum boosters come with a fitting that you connect the vacuum line to that looks just like a check valve but is really just and elbow.

Generally if you have a hard pedal it’s a booster/vacuum problem. If however the pressure required to push down the pedal seems normal but the car’s not stopping then it’s a brake system problem, which among other things could be …

Master cylinder needs rebuilding
Old rubber brake lines that are expanding
Contaminated brake fluid (water is compressible)
Pistons sticking in the brake calipers
Badly worn/grooved rotors or pads

So in your case if it takes both feet on the pedal to stop the car but the car does stop it sounds like a boost problem.

Screenshot 2023-05-16 at 3.44.09 PM

well I have a check valve which is good, but the red thing has me stumped, is that also a check valve?

I am used to seeing the check vale mounted on the top half of the housing, not the bottom - not sure what the red unit is - Tex.

David …

The red plastic 90degree fitting is how you connect the vacuum hose to the brake booster. It may or may not also be a check valve. I’ve seen both. Pull it off and blow through both ends, if there’s no restriction it’s just an elbow not a check valve.

As far as it being at the bottom or the top of the booster, while I may suck at a lot of stuff I am not a professional suckologist (one who knows a lot about how vacuum stuff works) but I can’t see how it would make any difference.

One other thing … most car vacuum booster systems have a vacuum reservoir to ensure you have maximum vacuum available at all times or in case your engine quits. I’m 90% happy with my modified vacuum booster system but I wish I had a little more assist on the pedal, a bit more sensitivity. I’m wondering if a reservoir would help with that.

just reading through and can’t see if david has posted a pic of where he’s taking the vacuum supply from on the motor, also disconnect the hose at the (red) end to the booster and check you have vacuum there when motor running…it all points to no vacuum at the booster

Looks like a used booster you bolted up?

I have had two cars now (past two months) come into the shop with rock hard pedals ala booster failure, however(!!), in both instances the plunger (one on the master side, one on the pedal) grenaded apart! The result being the plunger popping out of the master piston sleeve or pushing it laterally vs. in and out!

If you don’t have a somewhat normal feeling pedal with the motor off as you bleed the system, I would highly recommend yanking the booster out and making sure it’s mechanically sound over and above the diaphragm holding vacuum.

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Just some more info, I bought the vacuum assist kit as a kit all inclusive so I assume everything is new.

also the pedal felt normal when bleeding and moved plenty of fluid, only got hard after closing bleed screws.

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Caliper sliders not sliding?


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It appears to have rash and surface rust on the booster - unless you did the conversion many moons ago, those are definitely used parts.

I suspect you have a traditional (if you will) booster diaphragm failure if it gets rock hard upon ignition but otherwise bleeds and feels normal without vacuum.

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someone asked where I tapped in, also brake is hard even sitting overnight and not running.

David …

Did your brake pedal ever work normally AFTER you installed the vacuum system or was your problem a direct result of the conversion?

What company did you purchase your conversion kit from … it makes a difference.

Just to clear things up your brake pedal should be VERY hard without the engine running but not locked up. Even without the vacuum assist you should still be able to press the pedal about half way down although it will take a lot of pressure. This is actually a safety feature design in case the vacuum hose to the booster ruptures or the booster itself fails while you’re driving.

You can easily test your vacuum booster …

  1. With the engine off pump the brake pedal several times until it gets very firm (which will be about half way to the floor) and hold it there for about 10 seconds. If should remain hard and not keep going down. This tells you it’s not leaking.

  2. Now while still holding the brake pedal down start the engine. The pedal should immediately drop further. Now the pedal pressure should feel normal. This tells you the booster diaphragm is not ruptured.

is it possible that 1 stuck caliper could cause this? And the brake doesn’t change at all always hard, running or not.

Yes it worked fine until I replaced the reservoir seals and flushed the old brown fluid out and got nice clean fluid in, yes it was so old it was brown but the brakes worked fine.