Brake Booster Test

I have all new brakes on the car from PMB performance. I've only gone on a few test drives (still waiting for my plate... gotta love government bureaucracy) and I noticed a couple times it was very hard to brake.

Like I had to really stand on the pedal to get the car to stop.

I don't hear any hissing from the booster, and the brakes do appear to be working, mostly, I just don't know if I have a booster, master cylinder, or brake caliper problem.

I've bled the brakes, but wondering if perhaps there's still air in the system.

OR is there something else going on.

My usual test for a booster is to let the car run a few mins and then put my foot on the brake, turn off the car and see if the pedal rises right away.

It doesn't rise right away on the car, but it does rise faster than around 30 secs.

Just wondering if anyone has some concrete tests to run on the booster or MC to see if those are the issue.

Can't believe the cost of the booster/MC for these cars... well... LOL after 4 years I CAN believe it, it's just ridiculous.

So I don't want to pay the thousand or so dollars for a new one if I don't have to, if you get my meaning.

Thanks for any help.

 

Did you go through the bedding process on the new pads?

 

One thing to do is make sure that the check valve in the hose from the intake to the booster isn't sticking in the closed position (i.e., it should be easy to blow thru it in the direction that air is supposed to flow -- from booster to intake).

 

Yeah, IMO the check valve would be my first suspect.

 

If it is sticking is there any way to fix it?

 

Wouldn't hurt to try washing it out with some sort of solvent (if you still have the stock metal version). It might also be stuck open (so you only get good braking during engine deceleration) so check that direction, too.

 

I just checked it. I can blow through it but it takes quite a bit of force to do that.

The opposite side I can't blow through it.

Yeah I still have the metal one

 

If you are in gear decelerating the check valve does nothing. Vacuum source is constant. Check valve really only comes into play in stop and go driving.

Your standard booster test really is an invalid test. For a booster test take it on the road and test for braking effort under in gear deceleration. It happens so gradually they are way bad before most people notice it. It should easily lock up the tires.

Do a quick vacuum test on the hose. If its not leaking you almost certainly have a bad booster. Quite common.

 

I didn't trust my forty-year old booster and had it rebuilt at HarmonClassic Brakes with excellent results! Cost me around 500 in 2023.

HARMON CLASSIC BRAKES, LLC Receipt
1094 FIELDSTONE DRIVE CANTON GA 30114

https://harmonclassicbrakes.com/about/

 

A simple test for a booster (MOT test here in uk) is: engine off pump pedal 4-5 times- it goes hard. Start engine with foot on brake pedal- it should sink as the engine runs.

 

Ok looks like a bad booster.

Thanks for the info about Harmon brakes I'll give them a shout.

I'm thinking since I have to take it out (and I'm guessing that won't be too much fun) I'll go ahead and do the MC refurb too since it's probably not long for this world like most of the other parts on this car.

 

Replacement boosters for my Lancia were not available so I did rebuild it myself. The 308 unit is different, but there are some pictures and info here to see if that is the way you want to go, vs a new unit.

https://xwebforums.com/forum/index.php?threads/1976-scorpion-gold.43142/page-8#post-412746

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Thank you. I finally found a company that can do my booster and MC. They are White Post Restorations. I sent them off yesterday.

One of the other companies could do the booster, but not the MC.

After this part I do believe that every piece of mechanical part on this car has either been replaced or refurbished.

 

You probably considered this while bleeding the brakes, but would be good to confirm none of the hydraulic lines are clogged. My right rear hard line (the little 3 inch line on the caliper) was plugged solid - I'm planning to blow out all the hard lines with compressed air (and PCE) to ensure they are not clogged.

I've been considering rebuilding my brake booster (and M/C) as preventative since my car has been sitting for a decade. I don't have any first hand experience with these guys, but came across them online (if the above Harmon lead doesn't work out). Would be curious to know how it works out for you.

Apple Hydraulics in NY

http://applehydraulics.com/brakes.htm

Whitepost in VA

https://whitepost.com/brake-sleeving-rebuilding-services/

 

I sent my GT4 booster and master cylinder off to White Post based on previous mentions in threads. I was not very impressed with their service but both seem to work fine. After shipping them the units I heard nothing from them. After a few weeks I called and found they did not have the correct seals for the master cylinder. I contacted GT Car Parts and ordered a GT4 seal kit and shipped it to White Post. Heard nothing back so called again and they said the new seals were not correct either but they were working on another source. Heard nothing back so called again and units had been completed and shipped back to me. All seems good but wondered what would have happened if I had not been calling them to check status.

From others comments it seems Apple Hydraulics is more familiar with the Bendix components.


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