guido53

Hi to everybody
I bought my '75 Vette in 2005 and imported it from USA to Italy.
Since I bought it the braking feeling was not the best, so I changed: brake pads, all four flexible brake lines with new ones with metal braid, and all the braking fluid.
For ten years the braking was almost OK, but at the end of 2016 I started to get 2 problems:
- pulling to right when braking
- braking effect was good, but brake pedal travel didn't stop completely after braking, so in August 2017 I did following works:
- changed all calipers with reworked ones, O-ring type
- installed a new MC (Master Cylinder)
- flushed all fluid and put new DOT4 fluid
After all that the car did brake correctly, no more right or left pulling duing braking, but I still have the problem of pedal travel not stopping completely at braking end.
I mean that braking effect is good and correct, the car brakes strongly, but if I keep costant foot pressure at the end of braking action the car does stop correctly but the brake pedal doesn't reach a stopped position, it goes on slowly slowly till floor, this takes 5- 6 seconds after the car is already stopped, its not dangerous but it is strange and annoying.
I thought this woul have been due to a worn MC, but now it is new !!
I don't think this effect can be due to prop. valve, am I right ?
Could the power brake booster be the cause for that?
Any other suspect or suggestion for investigating the problem?

Sorry for my poor english and thank you to everybody.

derekderek

Bad master cylinder. Seal inside let's brake fluid return to reservoir. Maybe just a rebuild kit.

terrys6t8roadster

bazza77

You don't mention bench bleeding the new M/C at all in the large list of jobs done .

If you have just put it on and bled the whole system as one it might be the cause as air is trapped in the M/C no matter how much you bleed each caliper .Plus its an ongoing problem but the original one might have been plain worn out or full of crap and give the same symptoms.


I have read stories on here of guys getting a brand new M/C and it being no-good from the start .

So , possibly air in the M/C or its just s**t from the start. ???

On the subject , I have just finished rebuilding my back brakes on my 77 and I put new braided lines all round and I used 3 litres of brake fluid to get a good tight pedal back , it took about 7 times going around the car to get the air out. Tapping the steel lines with a rubber hammer and each join as well . But now the pedal is as hard as a rock !!!

HeadsU.P.

bazza is correct. Still air somewhere in the system. Pedal should not go completely to the floor even if the engine is off, which omits the booster as a issue.

Check into my photos of proper bench bleeding of the master on my profile, photo album.
I would pull the MC back off and start over. Bench bleed, then bleed all four corners again not touching the brake pedal. Use other methods.

MelWff

when you bled the calipers you went right rear outer, right rear inner, left rear outer, left rear inner, right front, left front?
You tapped the calipers as you bled them to dislodge air bubbles?

cagotzmann

I would check the following. To see if you find any air still in the Master Cylinder.

https://www.corvetteforum.com/forums...-cylinder.html

guido53

Hi, at first thanks to everybody.
I understand I need reworking some step:
- I did bench bleeding on MC, but now I'm no more sure if I put enough attention in doing this operation
- I'm not sure about the caliper bleeding sequence, I even didn't know about a specific sequence to follow
- I didn't tap the brake line at all, I didn't know about this trick..
So, next weekend I will repeate above operations using the right suggested process.
​​​​​​I'll let you know result

HeadsU.P.

That video works IF you have bleeder screws on the MC, not all yrs do. The bubbles are not leaving the MC unless there is a bleeder screw open somewhere. And thats tough to do with the tires still on the car.

cagotzmann

Just check you MC for the little holes at the bottom. The 75 MC will 100% bleed as in the video. My Wilwood MC even has these, and by design to remove air from the MC during normal operation.

You don't need a bleeder screw open anywhere to follow the procedure. Just "slowly" pump the brake pedal. Works everytime.

Works only if you get the MC at the right angle. Where the back of the MC is higher than the front of the MC so air is trapped right at the MC piston. You don't need to open any bleeder screw.

cagotzmann

Caliper bleeding sequence is not a must. Using the procedure where you bleed the longest line first to the shortest line, only reduces the amount of fluid required to flush a brake system.

If you are unsure about the MC, Try bleeding the MC on the car first. Using the procedure from the video you must see either air or fluid coming out of the holes at the bottom of the MC. But the trick is to make sure the MC is at the proper angle to make this work.

Peterbuilt

Disconnect the FRONT M/C line and plug the M/C.
The pedal should stay rock hard.

Reinstall the FRONT line and remove the rear line and plug it.
Again the M/c should stay rock hard.
If not replace or rebuild the M/C.

On the rear calipers bleed the INNERS first then the OUTERS.

HeadsU.P.

Keep in mind if you procede with the Peterbuilt method, you just introduced air into the system. So an entire bleed will be needed . . . . . . . again.

Be very carefull what you use to plug that MC brakeline port for testing. You can damage the flair inside using a bolt.

Peterbuilt

Yes, Guido53 will have to rebleed the entire system but he would still have to rebleed if he rebuilds or replaces the M/C.
Better to test a part than to change it for no reason.


The pedal going down slowly might be because of a seal in the power booster going bad.

Questions for Guido53:
Is the check valve failing?
How much vacuum does your Corvette produce at idle?

Doubt the 'prop valve' is the problem if it's not leaking.

Kacyc3

My 79 feels the same way, have never been happy with the brakes in my car, I was thinking about swapping to a different mc to get away from stock engineering and look for better pedal feel.

cagotzmann

I have switched from the OEM to Wilwood (after the oem failed) . There is no difference in pedal feel. Changing Brake Pad material is the only way you will notice a difference.