Might be missing something simple. 73 bronco everything has operated as designed for some time. Now while in gear and driving the brake pedal will bleed off and goes to the ground to where I have to pump a second time. I just completed a flush with a vac and nothing changed. Front is disc and rear is drum. All corners had fresh fluid coming from bleeder and nothing changed at the pedal. Possibly an issue with the booster? Any info is appreciated.
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Brake pedal bleeding off
- Thread starter gumby77
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No. It’s either a master cylinder failing and leaking down, or some other part of the hydraulic system leaking.
About 90% of the time on the system that’s been working, it’s the master cylinder.
But at the stage, you can’t rule anything out.
About 90% of the time on the system that’s been working, it’s the master cylinder.
But at the stage, you can’t rule anything out.
^^ This. I used to do brakes for a living. Short of finding a visible leak in the system, that symptom is virtually always a failing master cylinder.
Broncobowsher
Total hack
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If you are not losing fluid, it is an internal leak in the master cylinder. Need a new master cylinder.
Common problem after bleeding brakes. The extra stroke during bleeding pushes the seal into part of the master cylinder bore that wasn't normally used. Some crude in that bore cuts the seals. Now the fluid leaks past the seal. Harder brake pedal application tends to make things better, the seal is forced tighter against the bore. Light braking is typically the worse as the pedal sinking and needing a pump to reset the fluid.
Common problem after bleeding brakes. The extra stroke during bleeding pushes the seal into part of the master cylinder bore that wasn't normally used. Some crude in that bore cuts the seals. Now the fluid leaks past the seal. Harder brake pedal application tends to make things better, the seal is forced tighter against the bore. Light braking is typically the worse as the pedal sinking and needing a pump to reset the fluid.
Thanks for the info. I’ll replace the master. Not leaking anywhere and everything was working. Was starting to think it was the booster and manifold pressures since I have a 351 with sniper EFI. Thanks for the save of going down the rabbit hole. lol
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Well, it never hurts to check those things anyway. Even boosters fail now and then.
But the booster alone won’t cause the pedal to sink, because it’s virtually a rigid connection between the pedal and your foot, and the master cylinder. If the master is still good, and every other component downstream is still good, then the pedal won’t sink.
So yes, they can still fail, but that is easy to test.
So when the engine is on, or off, and a pedal sinks, it could be a booster failure too. But you have to couple that with the master failing, or the pedal will still stop.
But the booster alone won’t cause the pedal to sink, because it’s virtually a rigid connection between the pedal and your foot, and the master cylinder. If the master is still good, and every other component downstream is still good, then the pedal won’t sink.
So yes, they can still fail, but that is easy to test.
So when the engine is on, or off, and a pedal sinks, it could be a booster failure too. But you have to couple that with the master failing, or the pedal will still stop.
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I’ve never had to try this, but I think a simple test would be to disconnect the lines, plug the output ports, and then press the pedal with both the engine off and on.
If it doesn’t bleed down either way, then the problem is elsewhere.
With the engine off, you shouldn’t be able to push very far.
With your foot still on the pedal, once the engine starts cranking the pedal should drop slightly. That’s normal operation of a booster.
If it continues to drop over time, the master is for sure leaking internally.
I don’t suppose that’s a definitive test of a booster, but if it passes the normal testing like that, it’s probably still good and that puts it all on the master.
If it doesn’t bleed down either way, then the problem is elsewhere.
With the engine off, you shouldn’t be able to push very far.
With your foot still on the pedal, once the engine starts cranking the pedal should drop slightly. That’s normal operation of a booster.
If it continues to drop over time, the master is for sure leaking internally.
I don’t suppose that’s a definitive test of a booster, but if it passes the normal testing like that, it’s probably still good and that puts it all on the master.
Ok great thanks. I’m picking up a MC tomorrow from local auto parts. Going to give it a shot before taking it out to Parker, az this weekend. Worst case is I will double pump while off-road and figure out the issue after the weekend. It still applies the brakes of course but when the pedal bleeds down about half way compressed I let off and pump again and usually I’m about stopped by then.
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Having just worked through some of your issues, I would seriously recommend double checking the plunger depth from the booster into new MC. Make sure the plunger is as close to contact with the internal MC valve. The process is described in detail in my thread regarding Brake booster losing pressure by DirtDonk, as well as a few of the various installation explanations on Youtube. Good luck.
It does not answer the question of whether your MC was bad or not, but at least that way you can eliminate it as a source of the other related brake problems.
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Great point JeepGuy! I completely forgot the most basic of modern parts problems.
The thing we mentioned previously, about not all parts being equal anymore, the depth of the piston varies greatly from one to another sometimes.
Not just the recessed versus shallow aspect, but the variations on each of those themes.
It’s almost expected nowadays that when you install a new master cylinder, you must recheck and readjust your booster rod.
Don’t assume that a new one the same as your previous one.
The thing we mentioned previously, about not all parts being equal anymore, the depth of the piston varies greatly from one to another sometimes.
Not just the recessed versus shallow aspect, but the variations on each of those themes.
It’s almost expected nowadays that when you install a new master cylinder, you must recheck and readjust your booster rod.
Don’t assume that a new one the same as your previous one.
Great call jeepguy thank you. I have yet to replace a MC so i was about to head over to YouTube university. The one i got waiting at the auto parts store is pictured with 2 different rods. I assume it’s a multi year fitment. The rods in the photo also look adjustable. Ill be sure to measure the one i take out and start there..
Driving or wheeling with a failing brake system is a bad idea. It is not going to get any better and you may run into a situation where the double pump does not work or you need brakes immediately. Make sure that you bench bleed the MC before you put it on the truck If you have not been following Jeepguy's post on his booster, it is a good read with descriptions of where problems in the brake system can show up.
thanks for the info. Was just watching a video on it now actually.
Next question is about the distribution block. Like i said mine worked since i owned it but curious if anything is needed when changing the MC or in the bleed process? Just bleed as normal?
Next question is about the distribution block. Like i said mine worked since i owned it but curious if anything is needed when changing the MC or in the bleed process? Just bleed as normal?
pcf_mark
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When in doubt this is such an easy test. You are taking the lines off anyway just plug then and try it. If the pedal stays firm you have a leak somewhere else but 90% of the time the M/C gives up. It is so annoying because you started off somewhere else then bleeding brakes exposes a sketchy section of the M/C bore and poof the seal fails. You can also loose the M/C with all lines still connected and see if there is brake fluid behind the M/C where it bolts to the booster or firewall.
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Somethings not right here.
A master cylinder with a rod already attached is for manual brakes. This is a deep well piston with a snapped in rod that would usually go directly to the pedal.
A master cylinder for a power brake booster, usually has a shallow piston well, and is meant to accept a short rod that’s already part of the booster.
So even if you are able to use the master cylinder, you will likely remove the rod completely, and use the rod that’s part of the booster. Maybe…
Do either of those also come with a spacer?
A lot of times manufacturers use a deep well master, but include a spacer “bullet” to fill the void when attached to a vacuum booster.
This is, as you say, for multi year and multi model fitness.
There’s no hard and fast rule that requires a particular master cylinder will only work with manual brakes, while another only works with power.
They can be retro fitted to each other, as long as the bolt spacing and register diameters are compatible.
As well as port size and orientation to fit the appropriate lines.
In many cases, a master cylinder meant for power brakes will have a larger piston diameter. But not always.
From the factory, FORD Broncos used the same size piston in the master cylinder for both power and manual brakes.
No difference to the master, but when used with manual brakes, they came with a spacer bolted between the master cylinder and the firewall to accept the standard rod.
What application did you order your master cylinder for?
Ok so shame on me for not realizing that someone prior to me owning my 73 did power brake kit.
Long day of working on this thing and running to the parts store I’ve got the “correct” 1976 brake MC on there. However when looking at the spacing on the plunger coming out of the booster it was all the way ran in and may have been contacting still just a bit when I bolted the mc to it. Anyway I put it together and bleed the brakes and took it down the street. The pedal is firm now for the most part but isn’t stopping as it should and will not lock up if I put my foot to the floor. I may try and find a skinnier nut for the adjustment rod from booster to try and get that little more spacing but not sure that is my issue.
Long day of working on this thing and running to the parts store I’ve got the “correct” 1976 brake MC on there. However when looking at the spacing on the plunger coming out of the booster it was all the way ran in and may have been contacting still just a bit when I bolted the mc to it. Anyway I put it together and bleed the brakes and took it down the street. The pedal is firm now for the most part but isn’t stopping as it should and will not lock up if I put my foot to the floor. I may try and find a skinnier nut for the adjustment rod from booster to try and get that little more spacing but not sure that is my issue.
Easy image insertion is a feature of being a forum contributor.
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You need to become a contributor. After that there will be a "attach files" tab under your post and it's very simple. Should be a sticky post "How to upload Photos" when you open up the forum here.
I had to double check my pedal throw, meaning how far it was actually moving to make sure it was what the MC required.
It sounds like you had to adjust your Booster Plunger depth into new MC? Or was that an observation on your older MC? Did you follow the playbook laid out by DirtDonk in my thread about similar brakes issues in post #14? https://classicbroncos.com/forums/threads/brake-booster-losing-pressure-with-engine-running.326727/
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