Brake Confusion

[medxsa13]

Hi, i just got my ‘76 C10 over from california a couple of months ago. Took it for its first test drive on the road a couple days ago and i’m so confused about the brakes:
- they felt really responsive and strong for the 10 minute drive to my friends
- i drove it home and the brake pedal was basically on the floor before anything would happen and super soft to push
- checked all the lines and none seemed leaking and the reservoir didn’t appear to have lost any fluid at all
- bled the brakes and no air came out at all
- when bleeding the brakes the first push would be so firm, the second softer and the third completely soft
- removed the one way valve as it seemed as though air was going both ways
- the brakes then felt super amazing and responsive again, and then suddenly went to feeling awful, soft and to the floor before doing anything

Could someone please help me and give me some advice because i’m at such a loss and not really sure what to do / what i’m doing wrong. apart from that it seems to be running so nicely

 

[PrairieDrifter]

Sounds like a master cylinder to me.

 

[medxsa13]

Sounds like a master cylinder to me.

what makes you think that? just want to learn and understand so i know for next time. the fluid in it doesn’t seem to be going down at all

 

[PrairieDrifter]

what makes you think that? just want to learn and understand so i know for next time. the fluid in it doesn’t seem to be going down at all

That's the problem. You aren't losing fluid anywhere, the usual cause of brake pedal loss. The only thing in the brake system that produces pressure is the master.

Brakes do weird things, I'm not really able to explain why they make pressure then don't. I'm assuming when it happens it's just recirculating back into the reservoir instead of pushing down the lines. The more pressure it builds with each tap of the pedal pushes the bad seal out of place till it blows out.

 

[HotWheelsBurban]

The fluid could be bypassing in the cylinder; going somewhere it's not supposed to and then flowing back into the reservoir when you let off the pedal.
But the symptoms you describe are either air in the lines somewhere (air compresses, good brake fluid does not compress)or a bad master cylinder.

 

[medxsa13]

ok this makes a lot of sense, thank you. i have a master cylinder spare for my 73 satellite, not sure if it’s a dumb question but could i use that one or do i need to buy a specific one for the truck? also is it better/easier to buy a new one or rebuild the one on?

 

[HotWheelsBurban]

ok this makes a lot of sense, thank you. i have a master cylinder spare for my 73 satellite, not sure if it’s a dumb question but could i use that one or do i need to buy a specific one for the truck? also is it better/easier to buy a new one or rebuild the one on?

No the one for your Plymouth is different. The square body ones shouldn't be too expensive. New ones are cheap enough that it's usually worth it in time savings, to install new instead of rebuilding the old one. Only time rebuilding a master cylinder makes sense, is if the vehicle is old,or rare enough, that a new part can't be had.

 

[medxsa13]

ok thank you for helping! will try and source one tomorrow and swap it over

 

[Frankenchevy]

Make sure the soft lines aren’t swelling too. Depending on your budget and if you can afford additional snafus from stuck line fittings, it may be a good idea to replace them as well.

 

[Grit dog]

^Yup may be on the short list to get the whole brake system up to date.
But yes generally sporadic, no external leaks, brake performance/ soft pedal issues are from the master cyl bypassing internally.
It pushes fluid from separate reservoirs to front and back. Same shaft, 2 different pistons with a seal on the shaft between the two. When that seal begins to fail it can/will bypass fluid from one reservoir or outlet to the other.

 

[medxsa13]

ohh ok yeah i understand i think, thank you for explaining it! never had one go before on my other car so was proper confused about it

 

[medxsa13]

put a new master cylinder in and it’s braking so well now! thank you for your advice

 

[75gmck25]

FYI - master cylinder has a relatively simple piston inside a tube that is pushing the brake fluid. One common failure on an old system is that as the cylinder heats up inside, a worn piston seal gets soft, and it no longer seals and pushes fluid. It may be fine when the brake fluid is cool, but not seal well as the fluid gets hotter.

Another common problem I’ve seen may be due to bleeding the brakes after a repair. If an old master cylinder has developed a ridge where the piston stops moving, it may be enough to catch and damage the seal. If someone bleeds the brakes and keeps pushing fhe pedal down until it hits the floor, that pushes the piston past the ridge and damages the seal. It’s best to firmly pump the brake pedal to build pressure when bleeding the system, but not stand on it until it goes to the floor.

 

[Trey Figgity]

he same thing is happening to me....I just bought new MC and same thing is happening still. when i bleed brakes the fluid coming out has not one single bubble....when driving pedal goes to floor....i have to push HARD and at very end of pedal travel if i push too hard it locks up the front brakes

 

[medxsa13]

he same thing is happening to me....I just bought new MC and same thing is happening still. when i bleed brakes the fluid coming out has not one single bubble....when driving pedal goes to floor....i have to push HARD and at very end of pedal travel if i push too hard it locks up the front brakes

did you manage to sort it?