View Full Version : Brake Issues
lenleg
06-11-2023, 07:38 PM
I am finishing up my MK4 but brakes are not working properly. We bled them several times which didn’t change anything. Just added a brake booster but still no change. With car off you can pump the brakes up to a firm pedal but as soon as you crank pedal goes soft. I rechecked plunger depth.
Any thoughts on what to check next?
Len
BEAR-AvHistory
06-12-2023, 05:55 AM
Did you bench bleed the master cylinders?
Railroad
06-12-2023, 07:02 AM
What brakes?
lenleg
06-12-2023, 07:37 AM
Did you bench bleed the master cylinders?
Yes ... we bled the master cylinder.
GoDadGo
06-12-2023, 08:08 AM
I don't know if this will help, but it worked for me.
Step #1
1. I gravity bled my brakes starting with the right rear first.
2. Once fluid started coming out I closed it off and bled the driver's rear second.
3. I repeated the process before moving to the front.
Step #2
4. Next I bled the right front first.
5. Once fluid started coming out I closed it off and bled the driver's side front second.
6. I repeated the process as I did for the rear and my brakes were firm.
Hope This Helps & Good Luck!
Regarding the sequence of bleeding brakes, Wilwood recommends bleeding the front and rear on each side simultaneously, as it may be difficult to get all the air our due to the balance bar. The recommend passenger side front and rear at the same time, followed by driver's side front and rear at the same time. Many folks have had no problems using the sequence GoDadGo recommends, but just additional food for thought that may help.
egchewy79
06-12-2023, 08:48 AM
I can't really think of why your problem is only present when you crank the engine. The brake system should be separate from any part of the engine set up unless you have a vacuum assisted brake booster.
I've had luck pressure bleeding if you can get an extra cap and put a Schraeder valve on it.
Norm B
06-12-2023, 08:53 AM
From your description, it appears that there is still air in the system somewhere. The power brake booster applies a lot more pressure than you can manually and is able to compress the trapped air more. Hence the soft pedal with the engine running.
A little more information on your brake setup would help to diagnose the problem.
Norm
CraigS
06-12-2023, 06:04 PM
If the booster youa dded is a vacuum unit this may be normal. If you have another car w/ a vac booster do a test. get in the car w/o starting the engine. Pump the brakes 4-5 time and notice the pedal gets firmer as vacuum is used up. Now start the engine, give it 30 seconds, and push the brakes. Notice the pedal now goes lower and is somewhat mushier than it was w/o vacuum.
Hoooper
06-13-2023, 11:07 AM
Yeah this doesnt make much sense with the limited details given, need more backstory. Did the brakes always go to the floor when running even when the brakes were manual (would make no sense at all)? Or were the brakes boosted before and you replaced the booster? Or is this a new problem since adding a booster?
lenleg
06-14-2023, 07:09 AM
Yeah this doesnt make much sense with the limited details given, need more backstory. Did the brakes always go to the floor when running even when the brakes were manual (would make no sense at all)? Or were the brakes boosted before and you replaced the booster? Or is this a new problem since adding a booster?
We have suspected air and bled both the master cylinder and brake lines before but after doing more research I don't think we used the proper techniques so we are going to try bleeding again. The brakes had a soft pedal to begin with and that's why we added the booster at the suggestion of Whitby. I will post on the results after properly bleeding for the master cylinder and the brake lines. Thanks to all!
GoDadGo
06-14-2023, 08:01 AM
Try the Gravity Bleed Method starting with the longest run first.
It can't hurt plus you can do it by yourself with no help at all.
That method cured my issues so maybe it will cure yours.
Good Luck, Happy Bleeding & Keep Us Posted!
cob427sc
06-14-2023, 01:59 PM
One other item to review: are the calipers installed on the correct side with the bleeder valves to the top? It is possible with some calipers, especially the rears, to be installed on the opposite sides. This orients the bleeder below the high point of the caliper piston. Every appears fine and bleeds fine except there is always a small poket of air in the caliper preventing a solid pedal from developing. The added pressure from the assist only makes this worse.